1) Formula for force (F) developed on a ballscrew for a given torque (T), lead (L) and efficiency (e) is:

F=2*Pi*T*e/L

So as you can see if the pitch of the screw, L, is increased the force is decreased , so going from 2.5mm to 4mm you would have 62.5% of the original force (common sense really...) However that's not really an issue since F is already huge - say the motor provides 1Nm of torque for the 2.5mm screw, that's F=2*pi*1*0.9/0.0025=2262N which is clearly plenty.

The real problem with using 4mm pitch instead of 2.5mm is you will loose resolution. One turn of the motor with a 4mm screw would move 4/2.5=1.6 times as far so your resolution will be 1.6 times worse. However if the ballscrews are driven from the motors via timing belts you could put a smaller timing pulley on the motor to compensate and end up with the same value for step/mm overall. You need to decide if you require the original resolution?

2,3) Considering how much more it will cost you to buy the correct ballscrews perhaps it's worth seeing if you can fit the cheap RM series ballscrews from linearmotionbearings2008 on eBay? The RM1204 ballnuts are the smallest he does.