I got Zwift last week and have done 9 events so far. On Zwiftpower it’s got me in A+ pace group.
I’m still learning on how the categories etc work (i’ve attached a photo of my stats).
I’ve noticed some events for A class say the W/KG is 4.0-5.0. I’m assuming if I get 5.1w/kg in any race that means I’ll never be able to do they events?
If there is no A+ category in the race, you should be able to race in regular A. (That’s certainly what the orange plus sign is trying to tell you as well…)
Also Zwift Power categories and Category Enforcement are not the same. They are two different systems.
If I get say 5.1w/kg for 20 mins in any race will that mean I can’t ever join the event I posted about? I’m obviously going to get stronger over the winter whole I do this but don’t want to only be able to do a few events!
No, A category goes to infinity power.
I would ignore that, I don’t know why some events say 4-5. It just means 4 and above. You won’t get frozen out.
Subtle brag! 
congrats on the fitness.
I think the A categories are just infinite.
Zwift has me enforced in A grade as well, but probably due to my high w/kg. I would be cannon fodder on most flat races, my sprint is barely more than 550w thanks to my physical problems from last year. 
If you are racing for entertainment value:
Create a new account, add a few KG to the profile and race B. You will find a ton more races fill with riders.
There is no reason to be hellbent on racing A, where you have no fellow races.
