First off, did you read the manual for your cooler (is it a box-cooler or aftermarket)?
In my opinion, 4000RPM is probably the upper-limit for that fan speed. It probably just can't go any faster. As for 60C, this shouldn't be too much of a problem if that's as high as it gets, and only if it gets that high under extreme loads. The fact that you've seen 26C indicates to me that it is receiving the proper amount of cooling. In fact, I'm more intrigued that the fan wasn't turning slower at that temperature. My fan will range from about 1200-4000 (Thermal Take Mini-Typhoon). So no, I wouldn't worry about 60C. Now, 80-90C, that could indicate a problem.