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None of the NAMC’s “Catalog of all Series Matchbox Models” (second edition), AIM's “1-75 Series Regular Wheels” catalogue (second printing, 1983), Stannard, Houghton, or Leake ("Concise Catalogue of 75 Series Matchbox Toys by Lesney" (4th edition, reprinted 2006)) lists any variation not already catalogued by Nick.
U.K. Matchbox examined the model in vol. 3 no. 3 ((May '79) p42 but did not list any new variations.

The plastic-wheeled 4b is listed with 9.5 mm front wheels. Here's one with 8.5 mm wheels on crimped axles, alongside a rather better version with 9.5 mm (actually 9.2 mm) wheels on rounded axles.