got-ya.. depends on how much u want to use it.. can be done im sure, but 4 gallons is less than 20l so after boil off and evaporation you will be looking at 12-15l beer out max with a standard ag brew, so if following a standard 23l recipe you will be using a bit of math or s/w to adjust the recipe..
u cld use hand nibblers to cut out the enamalled shell and fit a square box back to the element/s, ally wld be easier to drill the element hole with a cheaper bi metal holesaw and not need a qmax punch.
or depending on how much of the gas system u can easily remove u may even get access thru the bottom to fit a burco style element.
however might be more work than u want for something not so usable, a 50l ss french pot is only £40 or so when bought direct..
just my opinion tho,