In my experience with similar connections, boarding for AirNostrum in Madrid starts usually later than mainline flights (relative to departure time) - even if they still manage to depart mostly on-time. The bad part is that the gates that AirNostrum uses are at the edge of T4, so it takes quite a while to get there compared to normal Iberia flights departing from gates in the middle of T4.

I have ran many times from T4S to T4 K98 with that kind of short connection, and most of the times I have had to wait several minutes for the boarding to start.