Hi Rod,
to IIS!
There's a "golden rule" when it comes to the maximum practical magnification for a given aperture. It is:
50X per inch of aperture
Your 150mm reflector is a 6" scope, so its maximum magnification before the image starts to degrade is 300X.
HOWEVER, the atmosphere is the ultimate dictator of what can be used on any given night. Average conditions will see a maximum useable being 120X, regardless of how large your scope is.
Good conditions will allow 200X to 250X. Exceptional conditions upto 400X, but these are very rare and need an appropriate sized scope.
A 4mm EP will give you your maximum 300X, but the number of times you'll get to use it is very small per year. As Matt mentions, a 6mm would be used more often if you are looking for higher magnification.
I suggest you also look for EPs other than plossls. These see the eye lens (the lens of the EP that you look into) get smaller as the focal length shortens and the eye relief (the distance you need to place your eye from the eye lens) get smaller too. It gets to the point that you need to place your eyeball on the eye lens the size of a pinhole for a 6mm plossl.
The best short focal length EPs at a great price are the TMB Planetary Type II range. These can be found on Ebay too. There are other fine short focal length EPs, but these are also more costly, so the way you want to go is upto you.
One of my favourite EPs is my 9mm TMB! It gives me 250X in my 8" and 17.5" scopes (both have the same 2000mm focal length), and is my most used EP with the Moon and planets.