Part of the core issue in an RPG is to maintain an illusion of the game world being real.
Making a "real dragon" be something that your party of six level 3-4 characters can defeat has impact on the world game fiction. It means that a "real dragon" is only as dangerous as other things a party of six level 3-4 characters can defeat.
Mechanically modifying a dragon and making it defeatable is easy. Doing so while still making it "feel like a dragon" is a bit harder (especially in a game like D&D/Pathfinder), but still doable. Doing so without making dragons into wusses is the really hard part.
We could go with "the players got help from another combatant" -- they ally with another powerful being (an adventurer, or maybe another dragon) and fight it in a team. Problem is the PCs power level is so low that they may not be a significant help against said dragon. Watching two monsters fight each other isn't all that fun.
We could go with "dragon is crippled through some method" -- poison, bad habits, wound from an earlier fight. Making this enough that your party can reasonably win is going to stretch credibility. (Sample plot: they research, and discover the dragon drinks from a particular spring every week. They then poison said spring, and ambush the dragon.)
We could simply boost the player's power level. Give then 2-3 more character levels and a "real dragon" is quite fightable. A downside is that this sort of freebe power boost can cheapen other power boosts, it phase changes your game in D&D (spellcasters at level 6 are at the cusp of changing how the game plays, and stories you could tell before no longer make sense).
We could go with "the players have some situational bonus that makes it doable". For example, they quest for/are given dragon-slaying tools. An interesting problem becomes "why are they not useful against non-dragons?", and if they are, they can change the power dynamic of the game (see above). However, you could try to make the power upgrade be in a way that doesn't cause as much of a phase change: if it simply increases player's toughness and damage output without a massive utility boost (access to higher level spells) it could work.
As an example of this, the players find ancient magical battle armor. This armor boosts the player's size to large, boosts their stats, ups their weapon sizes, overcharges their spells (higher Damage/DC), boosts their saves/defences, doubles their HP and doubles the amount they are healed by magic, and counts as armor. The armor also comes with other things -- some of them have weapons, others have recharging wands, etc, one of them can fly for short stretches, etc. Basically, they gain mecha to fight in.
And mecha is awesome.
If you roughly double each character's effectiveness, that is worth about +2 character power level.
I like the mecha solution.
Armor of the Ancients
An ancient empire built magical battle armor its nobles fought in wars with. Most of this armor was destroyed, but the PCs have found a cache. The armor binds itself to the wearer and their blood kin: unless 40 years pass, no one else but a near relative can use a suit of the armor after it has been worn. The Armor comes in various different styles, with different effects.
All Armor of the Ancients has the following properties:
- It has a maximum of 1 charge per character level of the wearer.
- Your size is large when you are wearing the Armor.
- You gain a +2 enhancement bonus to all attributes in the Armor.
- You gain a +2 enhancement bonus to all saves in the armor, increasing by +1 every 3 levels beyond 1.
- It counts as a suit of enchanted armor with a +1 enhancement bonus for every 2 character levels of the wearer. The kind of armor varies by type.
- For 1 charge you can summon the armor as a standard action. It immediately provides defensive bonuses, but does not provide offensive bonuses until the end of your next turn.
- Your current and max HP is doubled when you have it equipped, and halved when you remove it (round down). If you remove the armor at negative HP, you are set to -1 HP. If you are reduced to -max HP (undoubled) while in the armor, you are removed from the armor by the damage.
- you can put the armor on in 1 minute
- every hour you wear the armor, or part thereof, costs 1 charge
- They recharge 1d4+level/2 charges at dawn, and at some other times determined by the armor
Armor of Night
- The DCs of the abilities of this armor are Dex based.
- It recharges at midnight
- As a standard action you can change the size of the armor to medium or large. For 1 charge you can do it as an immediate action. Weapons you carry are also resized by this.
- It grants a +5+Level circumstance bonus to hiding and moving silently
- The wielder can cast Knock, Invisibility, and Silence 15' radius as spell like abilities using 1 charge per level of the spell.
- You have true sight out to 5' per character level.
- You have a +character level enhancement bonus to initiative
- You gain a +4 enhancement bonus to dex in the armor, increasing by another +2 at level 5, 10, 15 and 20.
- Any precision damage dice you deal is doubled against targets normally vulnerable to precision damage. Against targets immune to precision damage, ignore the immunity and apply the damage without doubling.
- You can expend any number of charges to add +2 per charge to a skill roll after you know you have failed. The bonus cannot exceed your character level.
- If you roll a natural 20 on a skill roll in a life threatening situation, the armor recharges
- Counts as a Mithral Shirt
That should be enough to double a rogue-types power level. At higher levels more spell like abilities might be a good idea.
Other armor type suggestions:
Armor of Day
Fighter-types, with melee and taunting
Can create 1 magic weapon per character level.
Left hand makes force damage variant.
Right hand makes brilliant energy variants.
Charges can be burned to turn misses into hits.
Bonuses to jump type athletics.
Recharges at noon, and on critical hits in life-threatening situations
Armor of Moon
Burn spell slots to power features of the armor.
Boosts DCs of spells.
Recharges at moonrise
Provides little armor, unless you burn charges/spell slots
Provides enhancement bonuses to mental stats and knowledge skills
Armor of Sun
Empowers healing effects (maybe lets you cast some faster while making other attacks?)
Armor of Mountains
Melee-types, with aoe rumbling and toughness
Armor of Winter
Ice effects, riders on attacks
Armor of Spring
Self and Group Healing, mobility
Basically have fun, make some cool armor. Only reveal some powers on each suit, have them learn more as they gain levels. Characters are now adventurers who can "power up" and fight in cool mecha armor.