There are 182 4-letter words starting with R in the ENABLE dictionary — 5% of all 3,903 words at this length. R is a one-point tile, so the value in this list is positional rather than numerical — which of these you can play depends far more on where it fits the board than on what it scores.
The list divides into 7 groups by second letter, the largest being the 43 words with ra. Scanning the right group is faster than reading the whole list, which is why it is broken up below rather than run alphabetically.
If you know more than one letter, this page stops being the fast way to find it. Enter what you have in the unscrambler — the Contains, Starts with and Ends with filters combine, and setting both length fields to 4 narrows this list to the handful your letters actually allow.