From my short experience, even as a winning player multiple tables really kick up the variance in a shorter amount of time (condensate the swings into shorter time periods).
Currently on average over 12k hands I win around 22bb/100 hands. This is just to give perspective for the next part.
Yesterday I played 4 tables and:
1) In a 240 hands session was up around 1,1 buyins (44.83 bb/100)
2) In a big 1,4k session went from
zero to -4 buyins to +1 buyin to +0,3 buyins (2.5 bb/100 in the end but when down to -4 buyins it was some really ugly number
) while playing pretty evenly throughout and without tilting as far as I could tell.
3) In next 2 short sessions over 220 hands I was up 2 buyins (around 90 bb/100)
Basically, with more tables the bad beats can accumulate more quickly... I lost in a row 82% chance AllIn, 78% chance AllIn and a 45% chance AllIn and that murdered my winrate pretty efficiently. But by the end of the session I had one lucky suckout and few normal big wins and it all evened out (except, you know, I broke even over 1,4k hands so other than FTP accumulation it was a waste of time).
I think the best is to look at your overall success rate and if you think you are a winning player then just play your game and ride out the ups and downs. If you can't then take a break until you are calm again.
I did notice that when playing 4 tables I am a bit more passive and nitty than when playing 2 tables which I can do comfortably. Reason is, I can't focus on borderline stuff as much so I just fold those hands more.