Well we can't be certain on a definete start date but
Series one started on 26th March 2005 which was Easter saturday
Series 2 started on 15th April 2006 which was Easter saturday
Series 3 started 31st March 2007 and not the 7th April 2007 (Easter Saturday)so as not to clash with any other programmes(even though they were still delayed a week with Eurovision).
So seeing as in series 4 they seem to be filming way ahead of themselves then series 4 will probably air on 22nd March 2008(Easter saturday).
Some people say that Doctor Who will start when Torchwood finishes but if Captain Jack is going to come back then i doubt it will be in the first three episodes of Doctor Who, but if series 4 starts on Easter saturday =and if Torchwood runs smoothly Torchwood will probably finish within the first few episodes of Doctor Who, so hopefully at some point in time we will have Torchwood and Doctor Who to look forward to in the same week!!