Briefly mentioned in a magazine from the mid 80’s, there was apparently plans for a 4th game in the Spy Vs Spy series.
The third game ends with our two spys being blasted into space. Therefore the theme of the 4th game was to be set in a Space Station, which seemed reasonable enough. Most likely doing the same kind of tricks to each other, maybe a bit of gravity in there and other little tricks.
Sadly the game never materialized. After the 3rd game, it was found not financially viable to produce a 4th game, so nothing was ever started.
Richard Spitalny confirms in Retro Gamer #221 that no game design was ever started and the end of the 3rd game was a teaser, as they anticipated there was going to be a 4th game.
So another game we can close the door on with no chance of finding anything! Case closed!
Contributions: Wayne Womersley, Craig Grannell, Rory Milne (Retro Gamer)