I've been on 40 mg citalopram and switched to 20 mg escitalopram . will i have to wait 4-6 weeks for it to work or will it be a continuation ?

There is a good: chance that this will be pretty seam-less. Celexa (Citalopram) & lexapro (Escitalopram) are sister drugs with the same active component.

Should not take 4-6: As they are quite similar drugs, the effect should be noticeable much sooner than 4-6 weeks. However, assuming that citalopram was not working (hence switch to escitalopram), you'll probably have to wait 4-6 weeks before being able to write escitalopram off IF it does not provide any benefit over citalopram.

Escitalopram: You should not experience a lapse in benefit. It will be a continuation of benefit.