Whining noise when engaging 1st or reverse.

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.

Can you get it into reverse with the engine shut off? If so, but you can't get it into reverse with the engine running, your clutch may not be fully disengaging. The forward gears have synchronizers that allow the gear to engage even if the input shaft is turning a little, but brakes keep the vehicle from moving.
If you can't get it into reverse even with the engine shut off, see if it will go into 5th gear with the engine shut off. If it goes into 5th but not reverse, you have an internal problem in the gearbox. If it goes into neither, your shifter linkage (or the shifter itself in the top of the gearbox) may need to be adjusted.

Hi Heather,i have a 85 Nissan 720 with 2.4 engine and Weber carb.with 5 speed,so when you start and put it in 1st gear,does it do this then.

The synchronizer for that gear could be bad . Will it go into reverse with the engine off ? https://en.wikipedia.org/wiki/Synchronizer

if 4th gear is the only gear not grinding then there is a problem in the counter shaft bearings in the box
in reality there is no 4th "gear" as it is a straight through connection of the input and out put shafts and there is no load placed on the counter shaft set bearings

check the fluid in the clutch system (if hydraulic) (replace cable if manual). Bleed air out of system replace the clutch master cylinder.

Have you checked the level of gear oil in the transmission? Remove the fill plug and see if oil is up to the hole.

Is this a new truck to you? Some manual's have a manual reverse lock-out, meaning you have to engage the lock-out to shift into reverse. This is usually done by pulling up on a small lever just under the shift knob.

Otherwise, the lock-out is inside the transmission and if that breaks, the transmission has to be removed to fix it.

The other possibility is that you don't have a synchronized reverse gear. It will not automatically sync with the engine speed. Try lightly putting it in reverse while letting the clutch up to the engage point, you know, right where the clutch first starts to grab.

possible synchronizer in transmission is malfunctioing and needs to be replaced

A lack of gear oil in the transfer case or gear box will make shifting and gears become ruff then destroyed within a few hundered miles, sometimes much much less. Unless this truck has had a history of bad shifting technique I have found these trucks and gears to be extremely reliable. If the clutch makes a loud wirring noise when engaged and disengaged ther may be a bearing problem. Check the U joints in the driveline. Bad U joints would usualy result in a lot of gear box twisting and jerking when starting out but diminishes with speed.