What's the difference between a 2Ah and a 4Ah battery (for a cordless drill/screwdriver)?

I have bought a 20V cordless drill, with a 2Ah battery.
Now they also sell 4Ah batteries (that fit several other cordless tools in their tool line).
Can I use said 4Ah battery on my drill?
What would be the benefit, if any?

Thanks in advance!