If I remember correctly, one off a 40, 42 or 4430 fit the best on the 4020. The reason being, the 30 series Sound Gard had a separate section for the dash portion. On the later models, the dash is integral with the cab. For the 4020, the hole in the front of the smaller 30 series cab fits the existing dash best. Even on the larger models, simply because of the dash design, the 30 series again are the best. Not saying a newer one can't be done either. I have not mounted one as yet, but have seriously looked into it particularly for my 6030. Just commenting based on my findings. If you google a few things stuff should come up and even pictures of ones successfully done. I think someone even sells sheet metal and such made up for doing the conversions.
Ultimately someday I would like my avatar to look like this I forget where I saved these pics from, this is someone else's efforts here, not mine