Could be many things...three that come to mind are:
-underground critter ate the tubers
-those four plants had excellent drainage versus the rest of your plants, and are suffering from lack of water
-spider mite infestation

Glad the rest of your dahlias are cooperating with you this year! You've had a tough run the last couple seasons- may this be the best yet!

Thanks. It finally rained last night.

Thanks