The DVD set has only got ten episodes, yet it's being sold at the price a normal full series of 20+ episodes will be sold at!!! Also knowing our luck in the UK I doubt there will be any decent special features to accompany the DVD set, but I hope I'm wrong there.
I don't mean to put you off, but if you haven't seen the episodes then do buy it, season four is just amazing with new revelations all round, new alliances and the return of a certain 'boxed' Cylon (oh go on you knew she was coming back!). But if you watched it on Sky, it may be best to wait before buying as I'm sure the price will be reduced further when the second part of season 4 is broadcast.
As I have already seen the series, I'll be waiting till 2009 when the second wave of season 4 is aired and the complete boxset with complete seasons 1-4, the mini-series and Razor is released. Just so I can rewatch it at my leisure all over again.