I have two different 4 wheel walkers, one being quite an expensive, but lightweight , version from Sweden, which has a bag , and stick holders and a tray can be fitted.
The seat holds about 21 stone but you do need to buy a back support for safety, and the design keeps the brake cables in the frame so that they do not get caught up in things.
There are three or four versions available.
The other Walker is very much cheaper (from Aldi) , but more substantial and heavier, and I now use this on a daily basis, keeping the other for holidays, restaurants (remember them?) etc.
Both are easily folded and very very importantly, both have larger front wheels than rear wheels which makes it easier to travel through rougher terrain , sand gravel , and go over curbs etc.
I still have a three wheel Walker , which i consider to be quite dangerous if you have a balance problem, like me , or if you get caught in longish grass etc as it will tip over very easily- this is just my opinion, based on my experience, others may have different opinions?