I think the worst losing streak I have had has been 3 in a row, but I have never lost more than 4 out of 10. Team comp makes all the difference really. Played a match with Shaya for the first time yesterday, and had one of the worst possible team comps I think I have played in, and it showed.
But lagging and drops do cause a lot of issues too. The storms last night were doing all sorts of wierd things to my team, from complete drop outs to lagging and characters moonwalking. Fun times.
Edit: Getting really sick of playing the same 10ish characters in blind pick though. Draft pick will be all I play soon I think. If I play another Gangplank, Akali, Caitlyn, Lee Sin, Udyr, Graves, Singed or Nocturne again, I might give up playing.