Swapping for the RX-5/Cosmo brakes is easier.
A mate of mine wrote this out a while back.
At the rear, there are a few ways you could go. The RX-5 bits DO bolt straight into the RX-4 housing, but there's a difference in the bearing retainers which means that an RX-5 axle in a RX-4 diff can move in and out by about 10mm... obviously, this is not good...
1. Whole diff swap. Cut mounts off both diffs, then weld RX-4 brackets onto RX-5 diff.
2. Machine up a spacer to fit into the end of the RX-4 axle tube.
3. Get the bearings pressed off the RX-5 axle, remove the retainer plate and replace with RX-4 item. Press new bearings on.
The remaining options get too confusing to explain here (read the linked diff thread), but suffice to say that in Tweety's RX-4, we'll be putting a whole 121 diff in. But if we were staying with the 3.9s, I'd definitely be in favour of option #3.
Which-ever way you go, you need an extra bracket on the body, for the handbrake cable. Ideally, you'd cut one off a RX-5 or 121 bodyshell and weld it on, but it's a simple bracket that would be easy to make and can be bolted on without drama (it bolts on under the rear seat, so no-one will be able to see the welds/bolts from the top).
It may be possible to re-jig the RX-4 handbrake cables to work on the RX-5 brakes, but I suspect that it'd cost you more at the cable-maker than fitting the bracket... but it would have the advantage of looking totally stock.
thanks you, but is very rare a rx5 here