on most 4 speed transmissions 4th gear is a straight through connection, like the input is directly linked to the output, and all the speeds before 4th would be a ratio less than 1:1, so it makes me think that a previous owner has swapped out the rear axle to get a faster road speed. if you could take a few pics of the transmission and the rear axle that would help us indentify what you have to start with and possibly figure out the possible gear rations inside the trans and also the rear axle. does the trans have a floor shifter that goes into the top of the trans like a truck transmission would have, or possibly the shifter is bolted to the floor or the side of the trans and there are rods from there to small levers on the side of the trans in oreder to select a gear?
usually a pic of the rear axle pan cover showing the shape and the number of bolts that hold it on can be compared to an online chart to show what you have. it may have a tag on it that will show the gear ratio or it may be stamped into the axle tube if it is a chevy axle. are your wheels on the rear axle still 6 lug or have they been changed along with the axle?
here is an rpm calculator you could use
https://spicerparts.com/calculators/...rpm-calculator