It might be a little nerf after a team attains 4 dragons. However, 4 dragons should be way into the later stages of the game.
Basically, what the ranks of the dragon buffs do:
- Tier 1: Good buff to get for laning. Especially useful if you can manage to sneak an early drake and then go back to your business.
- Tier 2: Good for mid-game fights and skirmishes. Makes your team harder kill - time for tankies to shine.
- Tier 3: Did I hear rotations?
- Tier 4: In the late game stages. As you mentioned, negates the Tier 2 buff, essentially. If you manage to keep the other team from taking dragons at all (meaning, you're winning hard), enables you to win fights easier, ending the game.
- Tier 5: This is where it gets absurd. 10% Resistance/Pen? Ok, that's nice. 30% + 15% MS + strong DoT for everybody? This IS better than Baron buff.
It takes a minimum of 5 Dragons to get the buff.
Dragon first spawns at 2:30 and respawns every 6 minutes.
Assuming it takes a combined 30 sec for 5 Dragon kills, the earliest a team could get 5 stacks is 33:00.
Add 6 min per enemy Dragon and a few minutes of Dragon dance and you don't get any realistic times below 45 min.
If you get that many free dragons you should be able to finish the game without it.
In an even game it should never reach 5 stacks. It's pretty much intended to be the game winning play that breaks a 55+ min stalemate.