A 4S pack on a stock TREX 450 V2 with an 11T pinion should net you ~ 3400 RPM.

How do you know your HS is 1600?

What motor are you using? If you are wanting to run 4S, you should get a motor that was designed for it (lower kV rating). In addition, with 4S, you need to be running a separate BEC or purchase an ESC that can handle the additional voltage.

There is a lot of tweaking and configuration involved with converting to a 4S setup.

As to your high frequency vibration, where are you seeing it?