Could be that your selector fork bits are missing (or that the selector is not moving witht he gearstick) and that it will not change the sycro forward or back.
The selector fork for 3rd,4th is separate from the 1st,2nd and reverse ones. They are all selected by a rod and pin arrangement. the pin does the pushing and pulling of the top end of the fork. as you move the gearstick left or right the pin selects different forks.
You will possibly have to take off the gear housing and inspect the forks for wear.
Could also be that the selector ring has exploded and is in bits at the bottom of the gearbox.
check the oil and plug, if the oil is full of glittery brass coulourd bits then if would be fork wear, if the plug magnet is full of iron filings then some ring or spline or grear teeth are getting worn.
hope this helps.