UNO not in sync (COM4)

I have been working with arduino uno for about one week. I still have a lot to learn. when I open sketch and go to tools-serial port, there are usually two options (com1, com3). When I try to load a program I get a not in sync error. Three times (out of about 50 tries) the tools-serial port gives three options (com1, com3, com4). If I check com4 everything works fine. How do I get the com4 to stay in tools?

I am using windows xp home edition