They fit but you cant inflate too high a pressure or leave at inflated pressure as with heat they will tend to distort the rim

yacht is long gone, but the sail was rescued from undr atree and used on TP1's first mini, thus it has also gone but from use not abuse.
a 4.00x6 tytre will fit a 4.00x4 rim , but the important thing here is weight, you may need to cut/buff all the tread off to keep things light. you couldalso consider fitting some kite buggy tyres, or look for some worn 8"quad bike tyres
gizmo found some lawnmower tyres a few years back but they were gone on his next visit

Hats off to the Boffins...it rained not 20 minutes later, looks like an all day drizzle.

OK! Here's C10 worth from me.. Think about gpoing to your local flying Field and cadging some used Aircraft tyre.. I am still using them on Occam's Razor,,,,, Happily. Look at 6"x6" Rim sizes as Edwards Adelaide

http://www.edwardskartwheels.com.au/kartwheelsdirtracing/ They were $45 a couple of years ago so they are a good Cheap Rim. I had a blow out on the Cheap Plastic Barrow Rims a week or so back.. Blew them up using the Thumb as a Gauge (Rule of Thumb). You know thrust the thumb into the side walls until you think they hard enough, about an hour in the sum and KARRRRUUUUMPP! It tore part of the bead from the tire and wrecked the inner tube--------"BIG TIME"
Point being the Cheap Chinese tires are about as much use as Pockets in your Underpants..(Plastic Rubbish). Find the extra cash even if you have to rob yer poor ol' Gran'ma
Ron
PS; on soft sand you will need a lot mor Sail PSI to get moving.