As stated the best route would be to use the M50 from the M5 at Stencham services and then the dual carraigeway A449 from Ross to Newport. No need to touch A40. It also saves the Severn Bridge toll. Road very good however maybe slight hold up past Monmouth for traffic lights. M4 generally busy up to Junction 36 at Sarn Services, Bridgend, be aware of lane changes at Newport and Cardiff East - reduces from three to two lane motorway. Road works are ongoing at junction 36 on the new roundabout off the motorway. If you use this exit for the services use Sainsburys, off the roundabout - far cheaper fuel and restaurant than official services area. Also large shopping retail outlet at this complex. West of Bridgend speed cameras are on the motorway at the 50mph stretch at Port Talbot. No further problems at present west to Pont Abraham (end of motorway). From here dual carraigeway A48 and then A40 through to St. Clears. Normal roads from this point west to Tenby, Haverfordwest, etc.
Good luck and rnjoy your visit to south Wales.