once you're playing 4 tables you're not playing the opponents anymore, you're just playing the cards. and that will cut into your profits.
Maybe your profit per hand but by no means overall. I play a general player and dont keep tons of notes or anything. The average player(not any specific one just the table as a whole I guess) at 50 and lower buy in stakes is rather predicatable and easy to beat. If I move up into like 200 buy in and larger I will not play 4 tables, probably 2.

so what should regular swings for NL be? around 4 or 5 the max buyin?
Yes I would say thats about right. But yeah thats about your average swings I would say. Probably closer to 4 than 5. Of course like kool said you will have the bigger ones now and then.

Another thing I suggest doing since you seem to be fairly new is to bonus whore the shit out of sites. Then most of the time if you have a bad game your bonus always offsets it to take you back into the positive or even. You arent attached to your money as much as I think that makes you play better. Then by the time you are done whoring you will probably be good enough to not really need them anymore and then they are just like free cash.