My 4600 had an issue like this a few years ago. When you kicked on the starter, sometimes it would just create a nasty grinding sound, other times it would engage and start up. A friend recommended that I remove the starter, and engage it while not installed in the tractor. Put on a leather glove and press on the starter gear while engaging. He said that if the starter solenoid is bad, then it will be fairly easy to keep the gear from extending. A good starter would be hard to oppose. Turns out it was easy to hold it back. A new starter was found on ebay for around $100. With the new starter, this have never happened again.