You aren't going to want to use your full upload speed when streaming. You probably don't want to go over 3000 or 3500 kbps if your stream isn't going to be transcoded on the other side, and probably not above 2000kbps honestly.
If you're using VBR, then x264 will only use as much bandwidth as it needs to meet your quality settings. If you use CBR, x264 will try to stay around your target bit rate no matter what, even if it doesn't "need" to use that much.
You can edit your bit rate settings int he Encoding tab of OBS settings.