For your work you need to ensure the supply is suitable for the work you are doing and that the work you are doing is compliant, you also need to ensure that earthing and bonding is adequate for the protective measures you are using.
The 32A MCB you have stated at one point is supplying DB4 and you also seem to state it is supplying DB3.
If DB3 is supplied via a 32A type 2 MCB with Zdb of 0.70Ω and the cable incoming is suitable to carry 32A then this is compliant, except for the discrimination issues, which you cannot address.
You wish to add 4 sockets (circuits or individual sockets?), 2 spurs (presumably 13A?) and 4 lights (again circuits or individual lights).
If you are adding 4 individual sockets, 2 13A spurs and 4 individual lights then the supply could well be suitable for the load, depending on diversity since this is a shop it may have long term demand at full load.
The second DB to be installed in a greenhouse is more inconvenient, but you would need to ensure that the DB is protected against the external influences expected and bonding of the steel frame of the greenhouse may well be required, subject to appropriate assessment and testing. Therefore if that were the case then I would expect an additional bonding conductor would be needed.