When you say, "...the clutch is slipping out..." do yo mean it jumps out of gear (like Neutral?), or does it just start slipping like the clutch is bad, and then grabs if you let up on the throttle a little bit?
Reason I ask: in that clutch there is one special cork plates that has bigger tabs on the outside ends, and it is usually found on the top of the stack, first under the pressure plate. This is the plate that wears the most in the "F" bike. If it is worn to the limit, it will slip under power in the higher gears, and 3rd and 4th usually have the highest torque load under nominal riding. The synthetic oil, if you oil is not a "low detergent" type, makes the foaming worse, which helps induce the slip. The fix is to replace that plate: it is the type that has slanted-cut corks on the face of it.
If your oil is not the "motorcycle oil" made expressly for these kinds of bikes, the synthetic acts like a super-suds agent. If the oil is a mineral-based type or a diesel-rated synthetic, it will have low detergent qualities and your bike (and gearbox) will hug you for it...
Thanks as always hondaman! Is the plate issue the same in the K as the F?
And to answer your question, it's slipping to neutral, or more like a false neutral. After it disengages, up takes me to 5th, down to 3rd (once or twice back to 4th.) But, I wondered the same thing you're getting at. So tonight I took the bike out and if info from 1/4 throttle to WOT in 4th, it holds. On deceleration it will hold until the 2,000ish RPM range, and then slip out. Not sure if this changes your diagnosis 
There's a possibility here of the forks or gear dogs being worn, or the gearshift drum "lid" being loose. The extra power afforded by your recent rebuild may now be nudging the torque just higher enough that it now lets it pop out of gear, if the gears were barely engaging before. The mainshaft twists quite a lot on these bikes, especially in the upper gears under heavy throttle. This makes it shorter across the width of the engine: if it is barely engaging 4th and it starts to loosen up, then the dogs on the gears jump apart.
The shift drum has a round disc on the top of it that holds some pins firmly against the shift drum body. On the K4 and later bikes, Honda did not always peen the screw in the center of this disc, and it can unscrew a bit, especially if the bike ever fell on the gearshifter. This can let the pins be loose, and the gears that suffer most from partial shifts then are 3 and 4. It doesn't turn all the way to that gear if those pins are loose, during a shift. That's one possibility.
Another one is: if someone replaced the clutch (ahead of you owning it) and installed the cup washer that holds the clutch basket to the mainshaft backward (hollow side out, instead of toward the engine), then after a while the dogs wear on the 4th gear (and sometimes 3rd), letting it jump out of gear under power.
Those are the 3 main things I've seen that can cause it?
