Hmmm it is only hard to get in 4th gear? How about 1-3. All the clutch does is disengage the engine from the transmission allowing smoother easier shifts. If the clutch was causing the problem I would expect to see some issues in the other gears as well.
If it grinds going into 4th while moving and not while stopped I would bet on a synchronizer or other internal trans problem.
If it doesn't really grind but doesn't get into 4th, it is likely that worn shifter/coupling issues are involved. The shift coupler is easily inspected. Directly aft of the shifter coming out of the back of the tunnel is a metal tube with a coupler on it that connects to a shaft coming out the front of the transmission. Check for slop where the coupler head attaches to the coupler shaft, there should be very little and movement should be damped by the rubber inside the coupler. Check the forked end of the coupler where it attaches to the trans shifter shaft. The holes should be round, the coupling pin should be round, there should not be alot of slop. Wear here can cause all kinds of shift related problems. Since you can only push the shifter so far too much wear can mean that the trans does not move far enough to get one or more gears. Having difficulty selecting reverse is common here. There is adjustment at the coupler end of the rod. The coupler slides in and out of the rod allowing positioning of the shifter handle for proper throws. There is also a plastic bushing near the shifter end of the shift tube it is often cracked or missing inducing slop in the system (access it by removing the tunnel pan under the shifter).
The clutch adjustment is done at a swivels located on the clutch rod and the clutch cable. The clutch rod leaves the bellhousing forward of the right exhaust manifold. It attaches to a cross-shaft which attaches to a cable going forward thru the tunnel to the pedal. You need to get in the shop manual and follow the adjustment procedure. Clark's Covair parts and others have nice repro manuals if you do not have one...(to cover a 1963 you will need the 1961 shop manual and the 1962-1963 shop manual supplement).
Corvair guy since 1982. I have personally restored at least 20 Vairs, many of them restored ground up.