Ok, I've put my 4013 and 4518 next to one another and I can confirm with 99% confidence that the rear fenders are interchangeable. Honda uses different part numbers, but I think it's because it's sold as a partial assembly with the shift lever gate and maybe the transmission access cover plate included. Maybe some warning or towing decals, too?
Anyway, they are identical in every way. Since my 4013 is a beauty, but the drive clutch is shot and the idler pulley on the transmission drive belt literally 'has no bearing left', I think next winter I will swap all the sheetmetal parts from the 4013 over to my tired old 4518. Also, will swap my near new Ohtsu rear tires on the 4013. I bought this tractor cheap, just to get the mower deck to swap onto my 4518 tractor.
The last picture below shows both tractors together which explains why I want to swap the parts over to my 4518 !
The 4013 and the 4514/4518 have the exact same frame, sheetmetal, tires, steering gear and mower deck (except drive pulley assembly). They have different transmissions, engines, and 4013 has only two indicator lights in the dash; fuel & brake (4518 has three; fuel, brake, overheat). Both dashes have four indicator light sockets, just use different overlays over them.
The 4013 is a one cylinder Honda vertical shaft 13hp air cooled engine with a dual pulley on bottom of the shaft. One pulley drives continuously to the input shaft pulley on the transmission and has a guide pulley and a tension/idler pulley between the engine & transmission (mine has a bad idler pulley). The other pulley has a clutch drive assembly for the PTO that is similar to the 3813 clutch.
The transmission input shaft drives another clutch for the transmission that has 6 friction discs, similar to a motorcycle clutch.