The 5-point harnesses had been introduced by the time of my last Chippy AEF in 1974.
When WK507 arrived here in 1975 (ex Birmingham UAS) it had the four-point harness. It still has this and I don't like it!
When the Royal Aero Club of WA's VH-RWI crashed in 1997, both occupants were badly injured - they had both "submarined" out from their 4-point harness, exacerbated by them sitting on too soft seat cushions that totally compressed with the high vertical loadings involved (no parachutes obviously).