I wasn't happy with the feel of the brake lever of my '99 F4, so first of all I overhauled and cleaned the whole system: I've put F4i calipers, which I restored, cleaned, changed all the seals. I refurbished the original master cylinder with an All ***** kit. But I don't recommend it; Honda provides a new piston with its kit (unlike All ***** and Proxx), and the rubber seals are of much better quality. Although expensive, It's worth the price.
The braking feel though still wasn't great. So I upgraded the brake lines to Frentubo braided lines, which are very nice and well-made. This made a difference, especially in the rear, but I was still unsatisfied with the front.
Searching the forum, I found that the CBR 929's radial master cylinder fits the F4 without any mods, so I looked for one. The best (and cheapest) one I could find was from a 2002 CBR 959, which is very similar but has different internals from the '00-01 one. I liked the '93-'96 ones because they have a similar brake lever and adjustment wheel to the stock F4 one, but I could only find old rusty ones. So, I thought: for 50€ it may be worth a try. And boy, it was. After installing the MC and purging the system, I noticed a change in the lever action right away. A short ride confirmed the initial feeling: much better response, improved feel, and more aggressive braking. I'm really happy about the upgrade and can't wait to try it on the highway.
I also changed the fluid with Motul RBF 700, so that might have made some difference as well.
The only downside is that the reservoir sits quite high, making it a bit too visible from the front glass. However, this is purely a cosmetic concern because it fits perfectly and doesn't interfere with anything. On the contrary, it's sleeker and better fitting than the original one. Maintenance is also much easier since the reservoir is large and sits straight, whereas the old one was a nightmare to open and flush.
I'll post a couple pics so you can judge for yourselves.

 

Good job!!!

Any ideas on the MC & caliper piston diameters and how they were changed from stock to where they are now? Thx

 

Thanks!
I thought that this would be pretty straightforward to answer, but it actually isn't. The '99 CBR 600 F4 calipers have the exact same pistons and piston seals as the '99 CBR 900RR, so the master cylinders can be swapped no big deal, I suppose.
On the contrary, the '02 F4i has different part numbers to the '02 CBR 900RR. The pistons have a different shape and material. So it might not be a "no big deal" swap as for the '99. I gave it for granted, but now I have a doubt.
When I took apart the calipers, the pistons from the '99 600F4 and the '02 600F4i looked exactly the same diameter, but I didn't measure them. I'll try to remember this and measure them when I'll change the pads in the future; I also have the old '99 calipers and pistons in my garage, so I can measure these too.
As for the master cylinder, the '99 600F4 piston and seals are identical to the '02 600F4i: the kit has the exact same part number for both models. So, the diameter must be the same. I believe they are basically identical, with some minor changes.
I don't have any '02 CBR900RR calipers on hand, so I can't tell you if the diameter is the same.
I could remove the levers and measure the diameter of the two master cylinders ('02 900RR and '99 600F4) without taking them completely apart, if you need.