Your observations are correct when checking them at 50 and 60mph.
2500 at 50 and 3000 at 60 are about right.

Don't know what you mean by shifting into 4th gear?

Transmissions of that era started out in 2nd gear. 1st gear was only chosen by manually shifting the shift or by kickdown when starting off.

The M103's power band heats up at 3000 rev's plus. These cars are not that quick at low rev's compared to modern or contemporary vehicles.