It's not a bug, it's a feature. If you can't upload enough, you aren't allowed many connections, so you don't download too much...

There's a workaround, but - why would you want to do that? To use up your full downstream of 64k/sec while uploading with 4k/sec? 3 connections usually do just fine, if you give LimeWire some time (like five to ten minutes) to connect to the network properly. Then you will receive enough search results and you don't take up to many connection slots on the ultrapeers.