Have a look at this Avon document. Scroll to the last page where it lists tyre sizes against rim width. They say 3.50 max size for a WM2.https://www.avontyres.com/en-us/tyre-ca ... -us-lr.PDF
Based on my observations any clearance issues with a 4 inch would be between the sidewall of the tyre and the indetaion on the left side of the rera mudguard where it is profiled to clear the chain.
And, just for interest, if you look under royal enfield, which shows fitments for speedmasters/safety mileage/Roadriders, you will see the recommended tyre pressures. These pressures may be relevant to those who use these 'modern' construction classic Avon tyres on their Ariels. (basically for speedmasters front /safety mileage rear 25 front 29 rear (p.s.i).