email for manager of four points express sheraton London UK?

That could be pretty tough. I imagine hotel managers of chain hotels like that are pretty well shielded from emails from the general public.

If you just want to change your reservation to the other hotel, why not just cancel the Four Points reservation on the website, and book the other one?

Join FlyerTalk.com com and post the same request in the Marriott forum over there

Someone will probably know the trick to backing in to the appropriate email address

Sometimes the specific webpage on the hotel site will have an email address
for the specific hotel. That's probably as good as you can do for starters. Send
something there and request the manager's attention.

Any Google search you do has the potential to be out of date.

As an aside, it's not quite clear why you want to communicate with the manager
from your post, but I will assume it's relevant. There is a Marriott representative
on Flyertalk who has the handle "Marriott Lurker", which is less sinister than it sounds;
they visit the site and look for users who have gripes about service, etc, and try to
help them. But it sounds like you have not stayed there yet.

Looking at the negative reviews on Tripadvisor the complaints mostly seem to relate to the fact that the rooms are very small. I can't see how emailing the manager will change this. If you are happy with a small room then fine; if not, book a different hotel.