Pulse generators maybe? It is the only other thing I can think of , other than the coil or CDI/spark unit, that is cylinders 1-4 specific.
They are magnetically operated switches, so you can test them with the tip of your screw driver. It's possibl that your 1-4 pulse generator is failing and the heat is something that worsens it.
There is a small metal nub that protrudes out of the body of the pulser. When you pass the tip of your screwdriver over it you should produce spark on those plugs.
-Bike in neutral
-Ignition on
- Touch screwdriver tip from pulser tip to closest metal object (anywhere on the rotor)
Checking for proper clearance might be a good idea as well. The FSM has good instructions on diagnosing these parts.