I'm not being funny, but how long do you expect the journey to take you? We have recently done here to Portsmouth in 5.5 hours, excluding a meal break, which is about the same as your journey.
If you are only thinking of a meal break, not an overnight, then I would say Cardiff West or Sarn.
Tredegar Park Caravan Club site is right on the M4.
As has already been mentioned, none of the services on the M4 in Wales is very big. Magor is about the biggest and most modern but even that is not somewhere I would want to spend a night with a caravan. I am sure none of the Welsh services have anywhere for caravans and the lorry bays get pretty busy at night.
Given it a bit more thought and if you really want to stay in the services overnight, I would probably make Swansea West my first choice. I fancy there is more room there than the others, or Pony Abrahm right at the end of the M4. This is a small services, but very quiet. There is also a large Asda store at junction 32 but the car park can get busy at times, or just of the A4232 at junction 33 are Tesco at Culverhouse Cross, Asda next to the Cardiff City stadium and another adds and a morrisons in Cardiff Bay. With the exception of Morrisons all open 24 hours.
Another option if you are traveling at the weekend is to take a look at the south Wales caravan club website, www.southwalescentre.co.uk and see if they have a rally that fits your schedule. You can be assured of a warm welcome.
Oh, what's occurin. I won't lie to you, at the end of the day, when all's said and done, Barry Island's crackin' and there are a few half tidy CL's and CS's not too far away.