Mazda 2300 grindes when shifting into 4th

Probably issues with Shift Solenoids. You need a transmission specialist or an auto electrician with a pro grade bi-directional scan tool computer that can tell the solenoids to operate and confirm that they did. Or it is the powertrain module that is at fault.

If it grinds when trying to shift, it is the synchronizers. Rebuild. If it pops out of gear. there is a plastic part at the top of the transmission that wears out. It fits around the ball of the shifter. It is two pieces. You'll have to remove the floor plate and pull it up to access the bolts to remove the shifter and replace those plastic pieces.

Sounds like the syncro may b damaged. Typically a clutch problem would be noticeable in all gears, especially shifting into reverse when vehicle is running but not moving. Try giving a 2 count for engine RPMs to drop before going into third to see if that helps. Won't fix anything but might cut the grinding sound. Till you can get the tranny fixed or replaced.

don't shift into 1st until stopped and 10mph or less is your down shift range to 2nd gear from 3rd

possible selector linkage adjustment , loose or shifted

Your pressure plate must be weak and about to let go.

Bad syncro for third gear. Sorry, but you aren't fixing this one yourself, unless you replace the transmission yourself.

Either your clutch is getting bad, or you're shifting down too soon, and up too late. Try double clutching. Push in your clutch and push it into neutral. Release the clutch, fan the gas pedal, push the clutch again and shift into 1st. All this does is sychronize the speed of the transmission with the speed of the engine. There were, in the 60's, a lot of cars with the old "three on the tree" that you had to do this every time, and you get pretty good at it after a while.

More than likely the synchronizers are going out,they allow you to move from gear to gear without damaging transmission.

check shift fork adjustment. make sure you bleed the slave cylinder.also check shift cable adjustment.