to be honest, i think 4tph to Temple Meads would be slightly ridiculous, but 3tph to Parkway, two on to Wales and one to Temple Meads wouldn't be too ridiculous (assuming 2tph running via Bath)
so you might well have, say
Paddington-Bristol (stopper): Reading, Didcot Parkway, Swindon, Chippenham, Bath Spa, Keynsham, Bristol Temple Meads
Paddington-Bristol (Semi): Reading, Swindon, Chippenham, Bath Spa, Bristol Temple Meads
Paddington-Bristol (Fast): Reading, Bristol Parkway, Bristol Temple Meads
Paddington-Cardiff Central (Semi): Reading, Didcot Parkway, Swindon, Bristol Parkway, Newport, Cardiff Central
Paddington-Swansea (Fast): Reading, Swindon, Newport, Cardiff Central, Bridgend, Port Talbot Parkway, Neath, Swansea