I would use "Descend 4000' QNH 1018".
If I don't want the pilot to bother me with further request, I would add "I call you back (in X minutes) for lower.
If there's a traffic below, I would say in an other message "I confirm, maintain 4000' reaching, traffic...", followed by the traffic information.
I also do it with flight levels, either for climb or descent.
Few months ago, there was an incident with a United States' aircraft.
After a callsign confusion, the controller said "XXX654, maintain FL310", the US pilot read-back "Maintain FL 310, XXY654"... and started a descent from 390 to 310 with no mention of a descent.
