Originally Posted by Playtennis
So if I play 4.0 with a strong 4.0 I'm doomed unless we win?
You aren't doomed, but yes, playing with a stronger partner raises the bar on your expected result, but that may or may not be a win.
If your opponents are also strong 4.0s you may still be expected to lose, perhaps fairly big, so a close loss could still improve your rating.
But if your opponents are weaker 4.0s, even as a 3.5, when playing with a strong 4.0, you may be expected to win. For example, say you are a strong 3.5 (3.45) and you play with a strong 4.0 (3.95). Your combined rating is 7.4 or average is 3.7 which is close to an "average" 4.0. If you play two opponents that are a bit below average (3.65), you would be expected to win (or technically, to win more games than the opponents).
But with the strong partner, you very well may win, and if you don't (or again, technically if you lose more games than the opponents), the computer takes it as an indication that you and your partner weren't as strong as thought and you'd be adjusted down.
If you were to play this same match but with a weaker 4.0 (3.65), your average would be 3.55 and you'd be expected to lose so a win (or even losing very very close) may improve your rating.