You can use as many elseif as you want. Whether it is wise, probably not as it makes it harder to understand the code, particularly if there's no comment to explain what is going (hint!)
Note that:
if A > B
minimum = B
elseif B > A
minimum = A
end
is more or less the same as
minimum = min(A, B)
The only difference is when A == B. Your code does not assign a value to minimum (so the variable does not even get created), whereas min does. Whether or not that was your intention or it's a bug in your code is unknown. If it was the intention then I would have expected a comment to state it (hint! hint!) so the question does not need asking.
Also, because of the elseifs your code will only calculate small_dist_R12 if it does not set a value for small_dist_R11 (so if YR11 == YR12), so presumably small_dist_R11 already has a value, otherwise it's a bug (if it's not, again, a comment is missing). Same for the other small_dist, they only get calculated if the previous minimum has not been calculated.
All in all, I think you meant:
small_dist_R11 = min(YR11, YR12);
small_dist_R12 = min(small_dist_R11, YR13);
small_dist_R13 = min(small_dist_R12, YR14);
small_dist_R14 = min(small_dist_R14, YR15);
if you really meant the code in your elseifs, then as I said comments are necessary.
Finally, if you are only interested in the final small_dist, and your YR are scalar:
small_dist_R14 = min([YR11 YR12 YR13 YR14]);
