1. Ditch the wired router for one of these wireless router click here or click here? page=1&store=8&cat=39&mfr=112&subcat=477 you can still connect the other 4 PCs to the ethernet ports of the wireless router if you want.
2. Or get one of this wireless access point click here and plug it into the port which you said <<One I dont need anyway, just a bit of backup>> with an ethernet cable.
Then you can access internet wirelessly with your laptop thru' this wireless access point after setting it up thru' one of the PCs first before plug it into the wired router.