I really enjoyed seeing the two books together in the same week, as they played off each other beautifully. I've said in the past that Morrison seems to be writing Final Crisis as almost Written Pointillism - small distinct powerful scenes designed to create a much fuller overall effect. Well, with Final Crisis:Submit, we see Morrison decide to show us one of those points, only go into great depth of it - giving us the notion that if we examined any ONE of the scenes in Final Crisis #4, we would see a story just like Final Crisis: Submit - poignant and powerful, only there are so many of them that we cannot even comprehend them all, giving the whole thing that much more of an epic scope.
It's a clever concept, although yeah, it would have been nicer if Submit had filled in the gap where we had no Final Crisis comics at all, instead of this silly "nothing...nothing....nothing....a bunch then another bunch" scheduling.
In any event, all of this is fairly familiar territory - it's superheroes fighting back after the bad guys win, only it's all executed almost note perfect (the only exception being that some of the dialogue could really use some work).
In any event, read Submit before you read Final Crisis #4. There probably should have been something to tell you that.