with the 4 gallon carboys, you would be best filling only 3 - 3.5 gallons as it will blowout if you try to put more in it than that.
brewing 3 gallon batches, in my opinion, that is not enough reward.
I would brew a 6 or 7 gallon batch in the plastic. Try to find a better lid, or, like many fine brewers, don't worry about getting an airtight lid on it. Just place the lid over the top so you don't get any crap in it, and let her go. Or, forego the lid and sanitize and place a piece of tin foil over the top.
Just my 0.02