4 weeks is long enough. Negotiating at the beginning is a better idea and keeps everything on an above board footing. Also, you should be asking for some arrangement to be made so that you can gain some remittance time to do the increased duties for the next while. To a certain extent part of the responsibility of the second is to step up now and again but if this is going from short to medium term then you need to take care of yourself to. Those who don't ask, don't get.