4th gear is 1:1 as you say with a 3 or 4 speed. At 60 mpr with 27" tall tires and 3:73 your rpm will be 3,033. Bit high anywhere. WIth 29+1/2" tires the rpm would be 2824. If it were mine, and I would not be off road driving pick a rear gear that would get the rpm down between 2200-2500 in high gear. WIth 27" tires that would be 3:08 or 3:23.
