I know there are tons of L plates for Arca Swiss compatible QR clamps, and there is a couple of manufacturers that makes L plate for Manfrotto RC2. But for RC4, you have to go with universal plates, and attach 2 QR plates. It will be heavy.
I had the Camdapter RC2 L pate previously (bought the first batch from Jim @ camdapter). I sold it to a friend who is using RC4. What he did, was to buy the Manfrotto 323 plate adapter from Cathay photo ($50), screw the clamp onto the RC4 plate.so the quick release becomes RC2 system. Or you can buy the pan head from Benro ($130+), and mount that onto your RC4 plate.
If you want to go Arca swiss route, the only way, is to either replace the RC4 clamp on your head (quite a bit of work), or you can get a Arca swiss style clamp with 1/4 inch thread and mount onto the RC4 QR plate. I recommend the Kirk clamp, that sells for $90, but it is hard to get, because it is always out of stock.
No probs Agree - better to convert Manfrotto to AS type of clamp or change to AS ready ballhead... more choices. I have also moved away from both RC2 & RC4 to AS. There is this company that I stumbled upon that makes clamps customized for Manfrotto https://www.hejnarphotostore.com/ FYI