How easy is it to install a wall light

It's difficult to say exactly without seeing it. It may require taking up some floor above to pick up the lighting circuit & the problem with that as you say property is 4 years old so most likely has chipboard floors which are a pain to remove. The simplest way maybe to fit a switch spur next to the socket you mentioned & to wire it from their but the trouble with this option is you may have to chase the wall out & have wall made good afterwards to get from the socket / switch spur to your light. I would get a couple of local electricians to give you a quote as they will be able to tell straight away how easy or difficult it is & their price will reflect this. If the wall is a stud work partition wall that could make things easier than if the wall is a solid wall which will involve some degree of chasing. You probably can have a dimmer fitted so long as the fitting you chose doesn't say otherwise. Some lights with built in LED'S for example aren't dimmable but it will say this on the fitting / packaging. Hope this helps

You could always ask/engage an electrician to advise or do the job. Impossible to say remotely.