It's not that I'm worried about discouraging you, more that I don't want to fill your head with numerous conflicting, yet logical and effective viewpoints. There is such a thing as too much information if you ask me (especially at certain points in the game).
You've pretty much just answered your questions right there with your comments about your diet. Do not, I repeat do not underestimate the importance of diet in this game.
My honest suggestion would be to read Berardi's "Massive eating reloaded" articles:
Follow the seven habits listed in this article for a solid two weeks (with a 90% compliance rate) and then re-evaluate. Have you gained weight? If yes, then continue. If no, then follow the steps that Berardi lists to slowly but surely increase your caloric intake.
One other really important piece of advice that I'm going to suggest is to get a lined notebook and make a food (or diet) journal. Write down each meal that you intend to eat and put a mark (for instance an "X") if you eat the meal, and a different mark (for instance an "O") if you don't eat the meal or if you eat a meal but one that doesn't apply to the "7 Habits".
At the end of the two weeks count up your "X"'s and "O"'s and figure out how well you've actually followed the plan. If you've got a 90% compliance or greater, than you are doing well and should continue with the process. If you are below 90%, then don't make adjustments to the plan itself, just try to follow it better.
I will say though that eating a "cheat meal" (one that doesn't comply to the 7 habits) is better than not eating anything.
As far as actual set/rep schemes, changing from 4x5 to 3x6 (and I'm assuming that you've unknowingly reversed set/rep schemes to rep/set schemes) isn't going to make a big difference. But, I would suggest using a rep range instead of an exact rep. So for instance you could do sets of 3-6 reps. If you can't even get to the bottom of the rep range, it's too much weight, If you go beyond the rep range, then next time increase the weight.
Could you possibly post what it is that you're currently doing for a routine? That might give us looking to give you some helpful advice a better idea of where you might need some help.
Hope this helps.