I have a level 8 Swashbuckler//Guild Agent Rogue. The Guild Agent archetype made me give up Uncanny Dodge. Is there a way I can do sneak attack damage to someone with Uncanny Dodge? Or is it still possible if I'm 4 levels above them?
Improved uncanny dodge says:
This defense denies another rogue the ability to sneak attack the character by flanking her, unless the attacker has at least four more rogue levels than the target does.
If a character already has uncanny dodge (see above) from another class, the levels from the classes that grant uncanny dodge stack to determine the minimum rogue level required to flank the character.
The most general rule is that rogues can sneak attack flanked characters.
Next most general is that you cannot sneak attack characters with improved uncanny dodge by flanking them.
Most specific is that characters with uncanny dodge can still be sneak-attacked by characters with a rogue level >= (sum of levels in classes giving uncanny dodge + 4).
Nothing actually requires the rogue doing the flanking to actually have any particular characteristics other than class levels as a rogue.
@Ifusaso correctly adds: Do note that your rogue levels must be 4 higher. So as a Swashbuckler 4/Rogue 4 you would not be able to sneak attack anything with Improved Uncanny Dodge. At S-4/R-5 you would only be able to use it on Imp Uncanny Dodge targets that are level 1.
Does your target have Uncanny Dodge (4th level Rogue ability), or Improved Uncanny Dodge (8th level Rogue ability)?
If your target only has Uncanny Dodge, then they have no protection against being flanked (and thus being Sneak Attacked).
Uncanny Dodge only prevents the rogue who has it from being caught flat-footed, and allows them to keep their Dex bonus vs invisible attackers.
But noting that you are at 8th level, if your opponent is a pure Rogue of the same level, then they would have Improved Uncanny Dodge. This does provide them the protection against being flanked, and prevents Sneak Attacks unless you are 4+ levels higher than them.
As others have also answered, the only basic prerequisite to beating Improved Uncanny Dodge is to have enough Rogue levels.
However, you should also note: this is only in relation to Sneak Attacks by Flanking. Other methods of using Sneak Attack may not be affected. For example, using Bluff to make a Feint forces the opponent to lose their Dexterity bonus against your next attack, making them vulnerable to Sneak Attacks.