Worn bearings which allow the input and output shafts to get misaligned during shifting will cause the problem you're experiencing. There is a "pocket bearing" where the input and output shafts meet. If the shafts and/or bearing rollers wear, the 3-4 shift collar will hang up as you're trying to engage 4th. gear. By wiggling the shifter around, eventually the shafts will self-align and the shift can be completed. It sounds like it's time for some rehab work with a "small parts" kit that includes all the needle bearings, gaskets, seals, etc.
Jerry

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