How about:
"Descend 4000 feet
The reason 'altitude' is inserted in RT relating to such clearances is not to distinguish from FLs (the THOUSAND bit ought to be a clue) but to avoid ambiguity, particularly for non-native speakers of english.
Example: aircraft flying at 6,000ft. Instructed to "Descend 4,000ft".
Does that mean descend TO ALTITUDE 4,000ft or could it mean descend BY 4,000ft (i.e.to 2,000ft)?
Yes of course WE all know the answer and of course WE wouldn't get it wrong...but as has been alluded to, these things are often learnt the hard way. Hence the inclusion in standard RT of all those things which so many object to (millibars, degrees, etc etc).