I'm pretty sure there would be enough differences to stop you using the IDE as is with the 3S, I can't remember details now but looked at it a while back and decided it would be a lot easier to use the 3X.
Likewise for the 4S but if you search you may find a thread where someone was going use that chip I think.
Another alternative may be one of the Mega644/1284 boards around, the 1284 IMO really hits power/size sweat spot.
Do you have to use Arduino, LPC make a great range of ARMs right down to 8 pins.
Rob