Four yr crop rotation question.

Link to John's info page

Towards the bottom of the page he says

crops like cucurbits and sweetcorn fill in the gaps

These come under "other crops" and can be poked in anywhere.

Both are good to follow late winter crops like brassicas, leeks and parsnips because they can be raised in modules and not planted out until May. Neither do they demand a fine seedbed which is sometimes difficult after those crops.