Fantastic trip, 4 days was c. 6hours/day riding (+lunch,breaks etc).
We got very lucky with the weather - only 20mins rain over 4 days - It's hilly though, so bring your climbing legs (1500m+/day)
The climbs in and out of Machynlleth were real biters, but the views! oh the views...
Road bikes with tougher tyres were good - apart from an hour of real gravel around the reservoirs above Cardiff most of the paths were pretty solid (wet weather might change that perhaps) - we circumnavigated one 4mile section between Newbridge and Llanwrthwl as we were warned that it was too rough for road tyres.

We happened to find the day when both bridges over the Afon Mawddach were closed for maintenance (the rail bridge at the Barmouth end is closed for quite a few months I believe) which resulted in an extra 25km back and forward, but at least it was very pretty.

We struggled to find anywhere to stay around Criccieth, so ended up in Nefyn across the peninsular, but again it wasn't an issue, and the back roads were equally lovely.

Getting trains to Cardiff and from Holyhead was fine - make sure you book your bikes however. This is much easier to do at a big station ticket office than on line or on the phone!