Haven't found an easy QlikView funtion way, maybe I have missed something.
You could always use
=num(([Date]+275-(MakeDate(Year([Date]+275-mod([Date]-2,7)+3), 1, (mod([Date]-2,7)+1) )-10))/7-0.5,'###0')
to calculate the week of financial year (starting April). Week numbering is in accordance / analogy to ISO-8601.
P.S: If you want a different start of financial year, replace both occurences of 275 with number of days from financial year start till end of calendar year (i.e. April 1st to December 31 = 275)
Hope this helps,