I was once given a rule of thumb concerning magnification and clarity. It seems to have held true. You have to follow some easy steps.
First know the magnification of your scope when using a certain ep. Mag is derived from dividing the focal lenth of the scope by the focal lenth of the ep. Thus with a scope such as yours which prob has a focal lenth of approx 900mm, you would divide that lenth by the focal lenth of the ep, in your case a 4mm. That would give you a magnification of 225X.
Next note the apperature of the scope. The apperature of you scope is 4.5" or 114mm.
Next, divide the magnification by the apperature. If the answer is greater than 1, (100 percent of the apperature) you are beginning the streach the ability to clearly focus or see the object. Concerning the 4mm you are using: since it produces 225X, when you divide the the magnification of 225X by the apperature of 114mm you get a whopping 1.97, thus you are at 197 percent.
The rule of thumb I mentioned before is that anything greater than 100 percent is going to be hard to focus. This of course is a rule of thumb. There are many varibles such as the object you are observing (the moon and some planets such as Saturn and Venus can almost always be observed at higher magnification), the conditions of the sky, the quality of the ep and on and on. (You ought to be able to observe the moon with the 4mm without much trouble. When doing so you should take care in using the focus knob in that the slightest turn produces great changes in the focus. If you have a problem getting the moon in focus something is wrong.)
I have a scope similar to yours and have often observed Jupiter at 4.5mm and 4mm, but more often than not I have to stay at about 9mm to 7.5mm which produce respectively mag of 100x and 120x. Those mags keep me around 100 percent.
I hope that helps.