Yes the most you can get with an Rx 470 4GB with Hynix memory is generally around 27MH/s.
I have been succesful in getting a few cards to run at 28.5 MH/s or so but most of them only run stable at 27MH/s or they require a higher voltage.
The issue with a higher memory clock like 2000MHz on the 4GB is that usually after a few hours or days, you will get massive amounts of hardware errors, and will require a restart. Usually the restart fixes the issue however you lose mining time during the restart. Hence you are better off just running them at a slower but more stable speed.
I suppose you have Hynix type H5GC4H24AJ, right? That should run 30+MH/s easy on good cards, but you probably use the timing from NerdRalph which is free and is also used in my tutorial. You need better timing. I've developed better one, but it was tested on RX 480 with few testers and doing around 31 MH/s, didn't cross the 32MH/s border as I wanted but good enough. PM me, I will mod your bios for free and let's see if it helps to you.