Hang on a minute. It may be absolutely spot on!
4 knots is going to be a fair old displacement speed at low - mid throttle depending on the boat size. Remember ribs (or anything witha transom) are not particularly efficient through the water when not on the plane.
£28 at todays prices is about 24ish L, so maybe 5 hours at maximum inefficiency on a 40Hp isn't too bad?
IF the engine is a 1990 model chances are it's going to be a traditional 2- stroke, which compared to a modern engine at that speed and load will drink.
So, some more questions:
How big is the boat?
When you were travelling at that speed were you creating huge waves?
Where was your throttle? (tickover, quarter open, half open,....)