When you say without a spotter, do you mean that if you know you have a spotter you can get more than 4, or you can only get 4 before they assist? If the latter is the case, you can only get 4 either way.
How much other chest work are you doing and how often?
The number of reps matters depending on your goal. I believe less reps is considered better for strength while more is better for size with the 6-8 rep range being a happy medium between the two. If you are looking for a pump, then you will probably want to do a little bit more than 6 reps, if you are looking for strength 3 to 5 over multiple sets would be better. It all comes down to work load. I tell you that you must lift 1,000 lbs. You can do this many different ways, doing 10 reps of 100 or 3 reps of 333 for instance, but the way your body adapts to each is not the same. This is important because what you want to get from strength training will dictate a great deal of the rep ranges you are looking to do for an exercise.
Also, sometimes numbers are psychological. Occasionally you can actually ADD five pounds and break through because you get fixated on a certain number. The mind is funny that way.