If you want to stay all grain, it's not unusual for brewhouses to yield lower volumes in trade for higher gravity. But 4% average is a sign of a very small mashtun, indeed. The most common systems I know of are build to yield 5% beer, most of the time. I hope you're getting high mash extraction from the grain you can use.
My first thought is to use Brewers Crystals or Malt extract to boost your kettle gravity.
Another thought is to do a reiterative mash, where you do two back to back mashes. The second mash gets the middle to last runnings of the first mash at dough in, which boost gravity significantly without too much volume.
My final thought is to ask why you can't just build a bigger mashtun? At 1 bbl, it's not too difficult or expensive. I happen to have a little side project of a 1 bbl pilot brewery, which uses a 150 qt (142 l) ice chest for the mashtun. Nothing fancy, just a slotted copper manifold on the bottom. I know I can fit about 90 to 100 lbs of grain in it, which should give me at least 9% beers.