I see no reason in general if $\operatorname{Ext}_P^1(4,1) \neq 0$ that, on restriction to R the four dimensional simple can't drop down and have a 2-dimensional submodule. I.e. On restriction to R it looks like:
\begin{equation}\begin{array}{c} 2\\ 2 \end{array} \oplus 1 \end{equation}
If you knew on restriction to KR the only irreducible submodule was two dimensional then this would be ruled out.