It will be the first time I’ve been to Wales. Could anybody enlighten me as to where any overnight spots in Wales would be suitable. I intend to do 4 nights and travel back to Scotland on the 5th day. Thank you.
Where in Wales are you going? There's quite a lot of it

As said previously Powys is van friendly many of the public carparks allow over night parking.
Ditto for the one by the Conwy in Llanwrst. as shown below which is a handy stop if cutting down from the A55.
I use the usual apps but I sometimes disappointed either the places are full or they don't meet expectations or the info is so out of date to be irrelevant. For example I am not particularly interested in sleeping in a layby by a busy road personally etc.
Peoples view on accessibility varies considerably for example if yours is a coach built think hard about some of the less accessible locations.
For example there is a side road near Beddgelert which is a great stop but driving to it is a challenge especially from the northern approach.
My van is tough I can bounce it off trees or dry stone walls and things without too much damage but a coachbuilt could easily sustain some serious damage. especially if the roads have 4-5' high dry stone walls on each side aka 'Sharp Teeth' Roads.
Google maps and street view (especially street view) is your friend, we use that and proper paper maps extensively to find spots not on the apps.
It does not mean we are the first ones there just that they are less likely to be occupied nor have the dreaded signs up!
Best of all lesser used spots tend not to be littered with other peoples toilet paper which seem to last for decades!
I always have a plan B and ideally three or four target sites, that way we aren't trying to find somewhere in the Dark!