Are there known 3- or 4-nucleotide RNA linkers with no RE sites?
I don't know the answer, but you could simply brute force the answer by generating all possible combinations of 3- or 4-nt length sequences, and comparing against the restriction enzyme database (REbase).

It would not take long to do. There are $ 3^4 $ and $ 4^4 $ combinations, respectively. Depending on your purpose, it might be possible when not considering engineered restriction enzymes (which are not likely to have recognition sequences found in nature).