I brewed a hefe Sunday and got terrible result. It was ment to be 1.056 end up being 1.042. Beer smith says if I add 2.5 gallons of a 1.100 wort to 3.5 gallons of the 1.042 beer it will brong it up to about to 1.052. Is this a good idea cconsidering I already started fermentation?
If you go that route you need to hop using the same schedule. 2.5 gallons is a lot of volume to make up for. I wouldn't do it though. I would just identify what went wrong with your process and redo it later.
I added to a pale ale that I made. It was a similar situation to yours and I just used about 1/2 gallon of water and a pound of dme. It really killed the hop character and gave the beer a homebrew taste. It could have just been that I knew it is in there so I thought I could taste it, but I won't do it again. You will still get a 4.x% beer out of yours which is still decent, and it should taste good!