Possible Brian, but the +4 became the same width as the +8 around '92 / '93. I'd guess that was when commonality of chassis occurred - certainly the M16 and later T16 +4s used the same gearbox as the +8s of the period.
Cheers,
Graham (G4FUJ)
Sold L44FOR 4/4 Giallo Fly '11 MINI Countryman Cooper D All4 '90 LR 90 SW