As per title, I should be renewing my Non Immigrant B in April

When I first renewed it, I got an unlimited re-entry permit or whatever for 5000 Baht

When I go to the Immigration office again next month, will I need to pay another 5000 Baht for unlimited re-entries again? Or will the first one still cover me?

I'll obviously ask at the immigration office while I'm there (Particularly as I'll be leaving the country 1 week after renewing my visa), but would like to know if I should anticipate the additional cost or not.

A multi re-entry permit is 3,800 baht not 5,000 baht so not sure what you had. A re-entry is only valid for the current permitted to stay time so any new extension would require a new re-entry permit if you intend to travel. As you do plan travel you will either need single entries at 1,000 baht each or a multi at 3,800 baht. Your extension of stay will be 1,900 baht.