My Mum has a Royal Python, we do not know his age, but he is just over 3 foot and about 850gm in weight.

He is currently in a 3' x 2' x 3'6" vivarium, but there are several problems, one is heating, it has a 150w ceramic heater with a pulse proportional thermostat and it struggles to get the temperature up down at low level, due to the height. Secondly the viv he has, being a tall one is on the floor and Mum struggles to get down there to clean change water, put him in and out etc.

So she is considering a more std size viv on a stand, the question is, knowing they are fairly inactive during the day, remaining curled up under a hide most of the time. although we are aware they are more active at night. Plus remembering he is not shut away and forgotten he is in her dining room, where she spends the majority of her time (tv comfy chair etc), and he comes out every day except the day aftter feeding, most days he comes out twice, and its not uncommon for him to come out 3 times in a day. The question is, 3 foot or 4 foot viv? 3 foot will fit in better and easier, but she can squeeze 4 foot in.

I know 4 foot would be the guide, but I think under the circumstances that he comes out and has the opportunity to stretch out regularly that 3 foot would be fine.

Id say go with the 3.
Royal Pythons are quite nervous snakes and too much space may cause stress.
I keep mine in a rubbermaid container, its great to help keep the humidity up, but doesnt look very nice
Four foot would be ok, as long as he/she has a nice snug hide to feel secure.

We have a Royal Python, but it is only about a year old if that, currently in a small faunarium (plastic tank), we are going to put him in a 36" x 18" x 18" Viv, with a heat mat, and a hide, and maybe a java tree type branch.
I would think 4' would be too big for a Royal..?