When to apply for a medical?

If your licence runs out on 18th April and you have a medical today the 9th March, when will your new licence become effective?
As soon as DVLA process it from 19th April or other?

I believe…
the medical is valid for four months from the appointment date?
you can drive, if DVLA has received your application, depending on medical findings?
it can take about three weeks to come back?

Thanks!

If I remember correctly the D4 medical start date is from when DVLA issue it

The duration of the medical is only twelve months this time. If DVLA has the application I believe it’s okay to drive, so I may apply closer to the due date. Thanks for that.