I don't have my 4000 here so I could check the presence of the ground
screw, but in any case there are versions that have both DIN & RCA
connectors for the inputs, and versions that have only DINs (like mine).
Perhaps the ground screw is present only in the versions of 4000 that
have RCA connectors (because you are not supposed to need one if the
turntable has a DIN plug).
In any case, the goal is to electrically connect the ground lead
from the turntable to the chassis of the Beomaster. If there really is
no dedicated ground screw anywhere, you can misuse a suitable mounting
screw on the rear panel. Another way is to use a spare DIN plug, connect
the ground lead to its sleeve, and insert it into a spare input.
Actually, if are using an RCA-to-DIN adapter now, you should just use the sleeve of that plug.