I have 2 reference books. For the Plus 4 model 1985 to 1991 one book quotes 153inches and another 153 in / 3886mm.
Cheers
Never rely on reference books for accuracy particularly for dimensions. a Plus Eight chassis is 2 inches longer than a 4/4 or early +4s until all models adopted the Plus Eight chassis. The overall dimensions quoted in reference books are affected by the type of bumpers fitted, whether they are fitted or not as they were optional extras in the 1960s and 1970s and whether overriders (early Morris Minor or the current style are fitted. Reference books often merely quoted figures quoted by the factory and these could not be relied upon.
It's a handbuilt car and blacksmiths' tolerances prevail!