Joseph,

Yes the 5.5 stands for the diameter (length) of the prop. The 4 or 4.5 is the pitch of the prop. The greater the pitch the higher the angle of attack that the prop is operating at. You shouldn't notice a big differance between the two props, although the 4.5 will probably have a little bit less thrust on the low end. If you want a little more thrust to start out with you might try a 6x3 prop, it should spin the prop at about the same RPM as a 5.5x4 and is pretty common.

Here is a sight that might help:[link]http://n-lemma.com/indoorrc/propcalc.htm[/link]

Happy Flying

Steve