If your 4510 is the same as my 4110 in the trans., the 4x4 shift fork is very easily bent and/or broken. It really isn't fork shaped, it's just a single rod that engages the collar of the 4x4 gear. The gear is splined inside and slides along the 4x4 shaft inside the trans and the outer gear teeth engage it's corresponding gear that is on the rear differential pinion gear/shaft - it has no syncronizers or anything and with the leverage of the 4x4 shifter it bends that single finger inside very easily. Mine used to get bent simply from my left leg resting against the shifter all the time. I heated my shifter with a torch and bent it away towards the fender more and haven't had a problem with my shifter fork since. I'm also very easy on shifting it, rocking the tractor back n' forth between forward & reverse to shift it in or out of 4x4 and being very light on the pressure I use on the shifter. Unfortunately you have to drain the hydraulic oil in order to remove the side cover with the shift fork to check it but it is very simple to check. You can use a screwdriver or similar to try to move the gear once the cover is off to see if it's OK. I've had to do a rear split on mine once to replace the rear bearing on the 4x4 shaft - it had disintegrated and I lost 4x4 because of it. Was one big job to install a bearing that cost me less than $4 at NAPA !! Hopefully yours is just the shift fork. I get mine from Galen at Lawall's and it's less than $20 if I remember correctly. Good luck !!!