To remain in compliance with your license, you will need to uninstall from the 4th PC and then install again and subscribe/activate with a different LiveID. You can then use this different LiveID for PC 5 and 6.
The current limitation is that each subscription is for up to 3 PCs and a LiveID can only have one active subscription associated with it.
Once you reinstall and subscribe with a new LiveID on PC4, your new subscription will become active. Your original subscription for PCs 1 to 3 will continue as it currently exists, apparently due for renewal in about 11 months. PCs 4 through 6 will be on a new 12 month subscription.
-steve