Cylinder filling

The Yong Heng is a 4350psi compressor according to the manual. I don't go over 4200psi as I don't want to blow more burst discs or stress the unit more than necessary. Still that is more than you are getting from the others.

I had the same problem with shops not wanting to fill to 4500psi and it is one of the main reasons I have a Yong Heng.

You might try some welding supply shops or their suppliers of nitrogen. I would expect there is someone around that will do it. How convenient they are or what the cost is another matter.
 
Have you tried a fire station?

You might also look up where to get the bottle hydro tested. They might be able to point you in a direction for getting it filled all the way.

Most fire stations won't touch them due to potential liability. My local dive shop fills my 4500 psi scuba tank. They like to have it overnight, to top off in the morning. It gets very hot in filling to that level, and overnight it will lose some pressure. 
 
Most fire stations won't touch them due to potential liability. My local dive shop fills my 4500 psi scuba tank. They like to have it overnight, to top off in the morning. It gets very hot in filling to that level, and overnight it will lose some pressure.

I understand the liability thing as an issue. It all depends on who's in charge of saying, yes or no. 

Jeez! If it gets to the point that they refer to it as "very hot" they're doing it wrong. You might consider looking into finding a different place. 

I fill SCBA bottles at 2 different fire stations. I generally fill at ~5 seconds per 100 psi up to 4700. They'll be "warm", at best, even when filling from close to empty. Then, they get stored either on the apparatus or on the spare bottle rack and settle out to 4500.