You could probably gain 10lbs EASY in 4 weeks...now if its going to all be LBM thats another story...you have to remember to about PCT and your gains AFTER PCT....
how much of those 13 lbs that you've gained so far is lean? typically, when people explode in weight in the first half of the cycle it's all water weight and fat (if they're doing a bulking cycle)... also, are you using an anti-e during cycle?
no anti e on cycle, i had to start letro because of gyno issues and that was when i weighed in at 13lbs more. I know letro keeps water retention out. So im guessing that im prob 8-10 lbs of lbm