Pitchers do stretch out further than 4 innings (based on pitch count of course) in their last start or two in real life spring trainings.
OOTP doesn't really have the concept of being "stretched out" currently though. You can take a guy whose been relieving all season and he can go 100 pitches immediately if you put him in the starting rotation.
There are a couple of overhauls I'd like to see in that regard at some point going forward
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In OOTP currently, though, I wouldn't recommend having them go further than the 4 innings. It isn't currently necessary as far as shaking of the off-season rust and spring training has higher rates of injuries.