Output Stage Clipping
There's a very good chance that the current limit circuit in the red box is restricting the high level output voltage under load: -

Q5 will be shutting off Q4 as soon as circa 7 mA is flowing through R3. This means that with a max of 7 mA able to drive Q11 (and assuming that Q11 has a hFE of about 100), the current Q11 can supply to the load is about 700 mA max. And, 700 mA into the load of 4 ohms is 2.8 volts i.e. it produces voltage limiting under load.
With a 50 ohm load, 700 mA is able to sustain +35 volts across the load.