Paid up-grades will get lounge access. Op-ups will not.
Serviceair does offer a pay on entry facility (should be £17.50 currently) but restricts usage to 3 hours. I think anyone would be bored here after 3 hours.

When is your flight? If you are travelling next spring, you might want to keep an eye on this website . It was meant to open this autumn but appears to have been put back. There is a free 5 minute Heathrow Express train service between Terminal 3 and Terminal 4. In the meantime, if you would like some comfort, you could try the Hilton hotel for the long layover (again at Terminal 4).

There are other airport hotels around Terminal 3 but you would have to take a hotel shuttle bus there.