You can get D4 form from most post offices, download it here gov.uk/government/publicati … ng-licence or your £50 medical guys normally have copies anyway.
When you send it to DVLA though at the moment it will go to the bottom of a very big pile and you are looking at weeks if your case is simple, months if not, unless you can quote some reason why you are a key worker and need it done for the good of the country.
If you are absolutely sure you have the entitlements on your licence and you have a clean medical you can drive under section 88 once they receive your D4 and if prompted they will send you a letter saying that. (Which is what I am doing now, albeit mine ran out in December and D4 went in just before lockdown.)
I already had a DCPC (DQC if like 
)card, on which all the courses had expired, and I was able to do online ones and bring it up to date and new card automatically issued.
Section 88 is here gov.uk/government/publicati … -with-dvla