The four ferry's where in the 1980s it must have been the Myrina that was the last launched from slipway
The 4 British Rail car ferries were the last launches from a conventional slipway.
`Galloway Princess` launched 24/05/1979
`St. Anselm` launched 05/12/1979
`St. Christopher` launched 20/03/1980
The honor of being the last ship down a slipway at Harland & Wolff, fell to the `St. David` launched 25/09/1980 from the Musgrave Yard. After that launch the Musgrave Yard was mothballed and put on a maintenance only basis. Around 1983/84 the cranes were toppled, the slipways filled in and it became a car park for workers of Bombardier.
Perhaps you are getting confused with the 4 Blue Star reefers that were built together in the Building Dock and floated out between 1984 / 85 ?
Here are a few pics of the Sealink vessels being launched.


The final Slipway launch at H&W, the MV `St. David`

Launch of the MV `St. Anselm` on 05/12/79