If the output curve won't go above 3600 or thereabouts then your inverter is holding back the panels. As I understand it PVI-3.6-OUTD-UK inverters with a serial number greater than 621395 are affected this way.
If you ask to have a 4.2 fitted your installer may need to apply under G59 rather than G83. No harm in asking I suppose.
If you ask to have a 4.2 fitted your installer may need to apply under G59 rather than G83. No harm in asking I suppose.