My tuppence:
For the time and money actually spent on the MR4, it'd have been better spent on coming up with a whole new airframe. Or converting an A320, or A330. If it comes to that, how expensive are used A340s to buy, operate? Ok, so that's getting quite large, but surely that means a few additional tanks can be added for very long endurance?
I've nothing against the P8 as such, it's just that it'd be nice to think that it's not just the USA who can convert an airliner into an MPA. Still, I suppose that a kitted out P8 is a single line purchase order. It should be really difficult to screw that up!
Anyone know how good the Japanese one is? Perhaps we'll find out, with military relationships with Japan seemingly blossoming at the moment.
Whose fault was MR4? Who knows. I do know that it's the hardest thing in the world to get someone to foresake their "old dependable" and replace it with something completely new, even when the old thing is obviously a pile of junk... At some point it's just not worth saving, and recognising the point optimally takes real talent.