Just be wary of how they'd want to calculate holiday if you did this - if you had 20 days on a 5 day week contract, if you went condensed hours to a 4 day week, some will then want to amend your holiday to 16 days, if they're calculating by day/week rather than by hours - so you'd still only end up with 4 weeks worth of days off in that scenario.
(Made up figures to illustrate the point, before anyone wants to get pedantic about 5.6 week legal minimums )
(Edit - this could also be dependent on how your contract is set up, i.e. is it by days with fixed times or by hours per week etc.)
Really useful to know. Didn't even think of that.