Specifically, I want to attack 4 times with Eldritch Blast as a 17th level Warlock. What is the easiest and cheapest way to gain advantage on all 4 attacks?
Play a drow warlock, and take the eldritch invocation Devil's Sight. Faerie Fire grants advantage against any creature illuminated by it. Devil's Sight allows you to see in magical darkness, and the Darkness spell grants advantage.
Other ways of gaining advantage:
From PH 194:
When a creature can't see you, you have advantage on attack rolls against it
If any of these apply to your target then you will have advantage:
- Prone (and attacker within 5')
- Cleric with Trickery domain. At level 2 they can use Channel Divinity to create an illusionary double. If both you and your double are within 5' of the creature then you have advantage.
- Paladin with Oath of Vengance. At level 3 they can use Channel Divinity for the Vow of Enmity that gives them advantage on attack rolls against 1 creature
- Rogue with Assassin archetype. At level 3 they gain the Assassinate ability and have advantage on any attack rolls against creatures that haven't taken a turn in the combat yet
Any spell that inflicts one of the earlier conditions or causes you to become unseen (eg - Blindness/Deafness, Greater Invisiblitiy) will give you advantage. These spells specifically grant advantage:
- Faerie Fire
- Otto's Irresistable Dance
Ways to get advantage
- Use inspiration
- Have stealth
- DM rules you have it because of something situational
- target is suffering from a condition that grants advantage
Only reliable option: On a previous turn (yours or a party member's) someone will have needed to inflict a status like restrained on the target.
The only reliable way to have advantage for that many attacks is to be making them against a target that is restrained or paralyzed.
I find it highly unlikely that even if 4 members of the party had inspiration that they would give it up to you for the nova round you are trying to build. Stealth + your own point of inspiration is still only 2 attacks. DM fiat is too situational to rely on and even still would probably only net you 1 use of advantage.