If I were you, I'd go with a 5 gallon cooler if you're not looking at going all grain. I do partial mash brews and boil on my stove. The brew I'm doing this weekend will use 8 lbs of grain and some of my recipes use more than that. One of the beauties of doing partial mash is you can mash as much grain as you can boil the runnings for, then your extract is just to compensate for the top up water.
And... you can still mash 4lbs of grain in a 5 gallon cooler, but you're not limiting yourself to that.
If you go with the 2 gallon, you'll just kick yourself later when you find a killer recipe you want to try and find that it takes JUST a bit more grain than your little mash tun will hold.