UPnP (Universal Plug N Play) is a function that allows you to connect through Firewalled routers easily, if your router supports it. I don't know with your router, but you should try setting Limewire to use UPnP. Maybe it will make it work.

In Limewire go to

tools<options<advanced<firewall and tick the (use UPnP) box, then restart Limewire
If it doesn't work you can always change it back

4 downloads at a time shouldn't cause problems

Sharing 900 songs shouldn't cause probems either, but it's a bit high number. The people that go after filesharers usually go after those that share 1000+ files as I understand, but 900 is pretty close to 1000 so maybe you should cut down just a bit

If you have some kind of security suite it will for sure have a firewall built in. Don't know if Symantec antivirus does though if it's just the antivirus. I have heard that the windows firewall isn't the best, so if you have another firewall then you should use it instead.

If you have the manual for your router, it should say if it is UPnP compatable, also if it uses NAT-firewalling. If it uses both then UPnP is the way to go.

Did your ISP provide you with your router ? Then they should be able to answer those questions as well, and tell you how to configure it