First, be PATIENT and don’t rush to purchase one just to have one.
Next, I’d consider waiting and finding a good #8. You’ll be able to find something usable for about the same money and the extra mass will come in handy.
#4 ‘smoothers’ are readily available and will make an essential addition to your collection.
Consider visiting the following web site: http://www.supertool.com/StanleyBG/stan0a.html
There you’ll find tremendous amounts of information on almost every type of plane Stanley ever made.
For my money, I have a #8 (jointer), #4 (smoother), #40 (scrub), and a ‘coarse ground’ #5 (jack) plane to serve as an intermediary between my scrub and jointer. Those are the four basics I have and I’d consider for you to begin. After that, you can search for the esoteric scraper planes, router planes, etc.
If you do get into hand planes, consider a good sharpening jig such as the Veritas model.
A sharp plane is a joy to use…a dull one, drudgery and frustration.
