If anyone cares any more, I own and use both OB1 and 4-Stroke in 500w versions. This is my opinion on comparison:-
The OB1 is their solid state amp that really tries to sound like their full-fat valve amps, with the typical limited Orange EQ range and the drive. It's great if you want a rack mounted item that you will not feel the need to change the valves in every couple of years, however it's no substitute really for an OR-120 or even an AD200 (have both also) if you absolutely want the traditional creamy Orange sound. It's close but not perfect.
The 4-Stroke lacks the drive of the OB1 but is able to open up a bit more in the EQ range and is more flexible on adjustment, similar to many other rack amps from the likes of Ashdown, Hartke, Ampeg etc. If you play all sorts of crazy venues and really get worried about your tone and insist on the additional EQ headroom then this might be a better bet. It does a good job at this and there are a lot of competitors out there but if you want a more traditional tone I'd recommend the OB1.