what to grow

Well done on what you've grown so far.

I suggest you mark out beds for rotation purposes.  If you want stuff producing over winter then you're looking at leeks and parsnips in the main.  Perhaps some brassicas.

The sowing and harvesting chart (there's a link to it on the Growing FAQs sub-forum) shows when you harvest stuff.

Organic matter is always good to add to the soil but general advice is not to use it before sowing parsnips or carrots.

4 beds = 4 crop rotation, Potatoes/Roots,  Beans/Peas,  Brassicas &  Onions.
If you don't want to grow as much of one of these groups as the others then fill in that bed  with the likes of sweetcorn & squash.
Salid groups can be fitted in gaps.
Manure and compost will improve the soil but make sure any manure you use does not contain any pesticides, see other posts.
Good Luck & Enjoy