On first viewing I was only able to spot the upper half as it went by ...And is that another slight Doppler audio effect to signal the form-up, a la 'Motorway'? Good stuff.
Now that is a very good ident. Really clever but the only problem I have with these idents is I think they should end on the '4' fully formed or do a Matrix style slow mo when the '4' comes up on screen.
As mentioned before there is a tip off of when the symbol appears as there is usually a loud sound. But some of them are too brief like the Motorway/road signs one.
And it also shows BBC1 once again how to have a long form up but still have time to show the ident. They still can't get it right with Mission Control can they?
I quite like the Rollercoaster idea although, admittedly, it's not the best in the set. I do like the way of enticing the viewer to 'spot the 4', especially with this one as it is quite tricky to see it. I only caught the bottom half of the logo the first time around as it was too quick. I agree with some comments that by going too fast can pose a problem because I dare say quite a few people will miss the symbol all together, after all I was watching specifically to see the form-up.
And it also shows BBC1 once again how to have a long form up but still have time to show the ident. They still can't get it right with Mission Control can they?
What BBC1 should really do is to bring in "zero" edits of all their idents; that is, versions which come in at the point of logo form-up. This would provide a more sombre intro for things like the news in the absence of a clock (though that excellent YouTube mock shows a timepiece could be done if they had a mind to!)