Since there is no "creep" when in gear and standing still and also the static shift are hard I think that there´s nothing wrong with the slave cylinder. Also since there is no scraping noise etc when You shift I beleive that the sychronisers are OK.
I also assumes that the shifter itself returns to the neutral position between 3rd and 4th as it should (push the shift lever to the left and release it). If this works the internal springs of the transmission should be OK.
My guess from Your description is that:
1) The shift adjustments is incorrect and needs to be redone (I´m afraid I dont know the correct procedure).
2) There is something in the rods/ linkage between the shifter and the transmission that blocks the correct function.
The best You can do is to put the car on a hoist and have a pal shift between the different gears while You look at how the linkage/ rods moves when they are operated and keep an eye on if there is something blocking them. Once You got that sorted out You can readjust the linkage. A common place for debris to end up is also below the shifter itself. Lift the rubber or leather boot and shift around in the different gears to see if there is something down there preventing the movement of the shifter.
I hope someone else on this forum can help You with the adjustment procedure.