There is no doubt it was one of the most brilliant moves in BB history. Dan was a dead man walking going into seclusion. Everyone knew it. He had no chance. Yet he managed to manipulate the entire house, creating sympathy for himself.
Then he convinced Frank to work with him. And honestly, I think it's in Frank's best interests too. Jenn will be loyal to Frank. She's never had an alliance with anyone in the game and now that she's made her bed, she's going to lie in it, even if the guy in it has crazy hair. Or maybe because he does. This move also gets rid of Britney, who other than Dan, is the one player who has both the strategic mind and the strength of self to pull off the strategy (Ian only has one of the two - I think you can figure out which one).
With Britney gone, we have Shane without his closest ally. We have Joe on an island, willing to throw in with whoever will get him another week. We have Danielle, who will do whatever Dan tells her to do. (How she could forgive his blatant manipulation of her emotions again... I just don't know. But she did and she will continue to do so. Dan has her under his spell.) And then there's Ian, who is not the master manipulator he thinks he is. He desperately needs someone else to be his backbone.
So if I were Frank, I would definitely want that situation with a strong 4 person alliance which can control things as much as any alliance can. Dan knows he can beat Jenn and Danielle in a vote and he is thinking he can probably beat Frank too (and after that move he did this week, he'll have an excellent argument to make to the jury) so he'll want to stick with the alliance. Frank has to be thinking the same thing - can argue he had the entire house against him and still made it to the end. So both of them would be smart to go the Final Four with this alliance.
The other option for Frank was get Dan out and trust Britney and Shane and Ian, who know they can't let Frank get near the end.
So really this was the best move for Frank and Dan.