The OS FSa-56 says 18% lube, is it ok if its a blend between synthetic and degummed castor? Or will this engine have problems with the castor?

A synthetic/castor blend fuel is OK. It will need to have some carbon removed from time-to-time.

I know how to clean 2c engines using kerosine or boiling and adding some detergent after removing rings etc..., how will i clean it from a 4c?

To clean a four-stroke engine, you use the same methods that you use to clean a two-stroke engine.

Would thre be a problem using a high oil quantity fuel like 20% castor / sinthetic blend in this engine? or is that just too much oil for a 4 c?

It's not "too much" oil. You'll just wind up adjusting the needles appropriately.