I'm learning British English. The r is usually dropped, so I never noticed a little thing, most of the time when I use an r before the th sound, my tongue does a kind of a tap or something between r and th.
When I say brother or brethren, for example (just with voiced examples, but the unvoiced do the same.)
I really don't know what it is called or if it is normal. My tongue does a tap down on my mouth.
I really am worried about this, because when I use American accent, it will be almost impossible. Saying 'More than four thieves' will be like a great challenge.
I'm sure I'm doing the r and th pretty well, because on th I spent five months practicing and the r exists on my dialect: southern region of Brazil.