it depends on what kind of r4 you have.
if you have r4ds it should say http://www.r4ds.com at the bottom of your r4 or nothing on it but if its r4 ultra it will say http://www.r4ultra.com. so if its r4ds you should go here to download:

http://www.r4ds.cn/download/r4ds.html
(copy and go and select language you want)

if its r4ultra go here to download:

http://www.r4ds-ds.com/pages/R4i-&-R4-Ultra-Kernel.html
(same as r4)