The only William Davies with son John in Merthyr Cynog in 1841 is a publican, so I've searched further afield.
I've found 2 farms with very similar names in Merthyr Cynog. In 1861 the Prichards are still in Tynwain which is in the hamlet of Dyffryn Honddu Lower. RG9/4208 folio 22 pg 5 He employs one labourer, a carter who rejoices in the name of William Ostrich (
).
In Yscirfechan, also part of Merthyr Cynog parish, there is a farm called Tynwaun RG9/4208 folio 13 pg 8
Thomas Davies head mar 30 farmer of 91 acres b. Llanfihangelfechan
Selina wife 27 b. Llandefalle
Jennett dau 4 b. Merthyr Cynog
William son 2 b. Merthyr Cynog
Emily dau 1 month b. Merthyr Cynog
plus house servant and a carter
I was rather happy when I saw the name Jennett and looked for earlier years.
1841 Vale, Llanfihangelfechan H0107/1368/6 folio 4 pg 1
William Davies 45 farmer
Jennett 40
William 15
David 15
Gwenllian 14
John 12
Jennet 8
Howell 6
Thomas 10
Roger 4
Richard 1
Elizabeth 2 months
all born in county
1851 Pylinglas(?), Llandyfaelogfach H0107/2489 folio 293 g 24
William Davies head mar 56 farmer b. Crucadarn
Jennet wife 49
William son 27 farmer's son
David son 25 farmer's son
Gwenllian dau 22
John son 20 farmer's son
Thomas son 18 farmer's son
Jennet dau 16
Howell son 13 scholar
Roger son 11 scholar
Richard son 9 scholar
Charles son 6
Mary Davies visitor 4 b. Talachddu
Mary Thomas visitor 70 farm labourer b. Crucadarn
unless otherwise stated all born Llanfihangelfechan
By 1861 William has died, but Jennet still on the farm with 3 sons, but not John or Thomas, RG9/4211 folio 53 pgs 15-16
Does your John always give his place of birth as Merthyr Cynog because it would be rather wonderful if he gives it as Llanfihangelfechan at least once? They're not consistent with ages and there's many a Davies as we know. If this family does look good to you, I think it would be worth looking for a will - that was a good size farm and it stayed in the family when William died.