Depending on the grain bill, you can probably pull off 4.0 gallon BIAB batches in a 4 gallon pot. Remember that the grains will displace quite a bit of water. 10.0 lbs of grain at 1.25 qts/lb of water is 3.96 gallons. You probably want to be closer to 3.5 gallons and watch your boil like a hawk, which would limit you to abot 9.0 lbs of total grain at 1.25 qts of water/lb.
Average (1.04-1.05 OG) gravity 4.0 gallon batches of ale will require about 8.0 lbs of grain, so you are going to be limited to beers under about 1.055 OG.
Find a 5.0 gallon recipe you like with about 10.0 lbs of total grain that's under 1.055 OG, then scale it down to 4.0 by multiplying all of the grain and hops by 0.8.
You'll then use your second pot as a dunk sparge after the mashing step, and may also need to top off with just a bit of extra water to get to about 4.25 gallons in your fermenter to yeild 4.0 gallons.
I'm a BIAB brewer...Let me know if you need any help with the process!