Not sure the infrastructure could support 4tph up the Marches, even to Abergavenny, currently. It goes to AB after Little Mill with a relatively long section. I think 3tph would be the max. I think it could manage 4 or 5, but that would need to include the Manchester, Holyhead, new Liverpool, and freight, before you consider this service. If the plan is 1tp2h for Holyhead and 1tp2h for Liverpool, alternating to give an a 1tph service to Chester, then it might just fit. But the shunt is still an issue.
The shunt at Abergavenny for terminating trains isn't exactly quick and simple, either.