I might have a group booking tonight with 4 guys. I will be stripping for one guy and having oral/sex with the other 3. The booking would be for 3 hours. Although I am unsure on how much to charge them.

My usual rate for a 3 hour booking is ?350 but that seems too low considering how mentally/physically draining a group session would be. I was thinking of asking for ?500-?600 but then part of me feels like that's asking for too much. 

If anyone has any advice on how to price group bookings I would really appreciate it!

Yep.

What you charge is up to you, but they are asking for a lot and it will probably mean you can't do anything for the rest of the day.

Your offer is incredibly reasonable, if they don't go for it it's because they are wasting your time. ?650 divided by 3hrs for the 3 is round about ?72 per hour for each of them - they're getting the bloody deal of the week IMO.

Plus as you've already identified, it is likely to be much more emotionally/physically demanding, you will need to be super alert for the whole 3 hours. I'm not brave enough to do it but if I was I'd want more. I'm sure that you risk assessed the situation to your satisfaction?