Fit straight on, just need to fit new hoses.
In my opinion not that much better than 2 pad calipers depending on pads, and before you splash out on 4 pad brembo's price the pads up first as they could set you back over £100 for decent pads.
much better upgrade is radial master cylinder.
I run 4 pad calipers with brembo pads and radial master cylinder, but didnt notice much difference until the master cylinder was fitted.
most bikes run radial brakes and master cylinders now so any should fit with minor mods, if you use one from an o4 on RSV it fouls the clocks on full lock so you will need to mod the lock stops.
obviously radial caliper won't fit!
If you only ride on the road leave them alone as they are plenty good enough.
I also switched from 2 pad to 4 pads. The diference, in my expirience, is that 4 pads bite harder at the beginning, while with the 2 pads you have to squize the lever a bit more, but in the end the braking power is the same. And how progresive the brakes are can also be sorted with diferent pads.
So In my opinion, if you get it cheap change it, otherwise dont bother.
(rsv4 went "back" to 2 pad setup)
There was good article on PB last month about the effects of new master cylinder vs. better brake pads vs. caliper swap.
There was about 10% shorter brake distance by just swapping new EBD HH pads. Of course there was further improvement with new calipers, but on this occasion the bigger master cylinder didn't actually help so much (the bike was gsxr K1).