On the outbound, remaining airside is not relevant because OP apparently has checked luggage which he will have to claim and then check-in. Thus, he can't remain airside.
It's just a rule of thumb, but I tend to do 3 x the MCT for separate tickets. Is that enough? No way to predict. Bad weather on the BOS-WAS corridor means that the micro-haul flights get cancelled while the long-hauls go out. Four hours might be more than you need or way too little.
On the return, I would worry a bit less. While TK won't cut OP any slack, both AA & B6 can be decent about missed flights and may rebook. The cost of getting to PHL is also a lot less than purchasing a new ticket on the outbound.