I thought this was normal for 400s on the tailights? The 99 I used to drive and my 95 both behave that way. Same bulb fires for turn signal and brake if I'm not mistaken.
On the brake with turn signal, one brake should flash when signaling (LH flash and RH solid for left, opposite for right turn).
It is normal. These trucks prioritize brake lights over hazards, so if you hit the brake pedal while the 4-ways are on it'll cancel the 4-ways until you take your foot off the pedal.