I am a high 3.5/low 4.0 who plays both levels and the thing about 4.0 is, that these guys know how to win the important/crucial points. I can be competitive with most 4.0's and never get blown out, but winning can be difficult. It seems like they know what to do when they need the point.
They are better at adjusting and figuring out how to win as the match goes on. It's almost as if you have to "learn how to win" at that level before you can start doing it yourself.
When playing at a higher level than you are used to, everything is magnified....your weaknesses are exploited and you must take advantage of your opponents weaknesses. Most
of these guys will win the long rallies, so you then need to go for more and shorten the points. Try different tactics and find something that works. If you sit back and let them play
their game against you...you will lose.