A standard electric fleet for all electrified parts ValleyLines network would probably have been sensible if no street-running was intended; but that doesn't necessarily mean FLIRTs which, being fairly complicated tri-mode units, might not have been the cheapest option in that case. But thank you for agreeing with me; they can do heavy-rail 4tph to Rhymney so there is no reason (bar the Taffs Well depot (which presumably will not be equiped to fuel/maintain diesel stock) and possible guaging issues) why they couldn't also do that to Merthyr, Aberdare and Treherbert.
There are currently toilets on all trains working ValleyLines services; therefore removal of toilets is a reduction in the standard of service compared to what is currently available and is hence a cause for complaint which might not arise were this a brand-new light-rail system. However, my concern is not just for the current network but also the prospect of future extensions; most likely from Aberdare to Hirwaun but with my 'ambitious' hat on idealy Aberdare to Swansea via Glyn-Neath and (even more ambitious) Merthyr to Brecon. These extensions would add to the journey time and could potentially run at a reduced frequency compared to the core (even with my ambitious hat, I envisage only 2tph between Aberdare and Swansea and the same to Brecon), increasing the need for an on-board toilet.
As for the T4, I used to be of the opinion that it should not run into Cardiff but terminate at Merthyr and only changed that viewpoint when I discovered it might actually be faster by bus than rail. Even so, at the moment changing on to a train at Merthyr and using the toilet on the way into Cardiff is presumably an option; the Citylinks are likely to change that. Also, a consultation done several years ago (2010 I think) regarding the long-distance bus network that is now TrawsCymru (which includes the T4) found that on-board toilets were a high-priority, along with improved legroom (neither of which happened). Unfortunately, fitting a normal low-floor bus with a toilet would be neigh-on impossible and there doesn't appear to be a sufficient budget to provide both a local bus service and a high-floor express coach. The tighter bends of the road network would also highten the risk off being thrown off the loo while trying to use it too, compared to rail. With a heavy-rail service on the other hand, there is an established solution to the provision of on-board toilets. The best excuse for the lack of toilets on the bus is "it can't be done"; that doesn't apply to rail. So pity the people setting off on the T4 to Newtown indeed; and try and retain the option of a toilet-fitted train at least as far as Merthyr so that they do not need to travel quite so far without access to a toilet. Even better, extend the railway to Brecon (with toilets) so they only need to brave the (toiletless) bus from there to Newtown.
Even if they do initially, there is no guarantee that they will do so for long. When I was using the Cambrian line roughly every fortnight the toilets at Aberystwyth were closed after 17:30 Monday-Friday (probably Saturday too), despite trains running at least until 21:30 and buses almost that late, and all day on a Sunday. I thought that was poor, but yesterday I saw that they now have a notice saying they close at 15:30.