I'm afraid I don't have any experience of the 4 speed box. I have heard the gearchange described as 'agricultural at best', but I'm sure it should be better than you are describing!
As you mentioned in your first post, I would check the clutch adjustment first - it does sound as though the plates might not be separating far enough. This, as far as I know, is effectively the same as the later 5-speeds I'm more familiar with.
Clutch adjustment can be divided into two operations: 'coarse adjustment' and 'fine adjustment'. The coarse adjustment should be carried out with the cable disconnected at the engine end, and the steel sleeve screwed into the clutch cover removed. If you remove the round plate from the clutch cover (held on with 3 screws) you will see a steel plate beneath it, which locks the position of the external part of the clutch arm's helicoid. This plate and the helicoid should be rotated so that the distance between the hole for the cable nipple is 11mm from the edge of the clutch cover.
When I had a look on Google for some photos I came across the following video link, which makes it much clearer than my description!
https://youtu.be/6ptXHE2i0x0?t=50m13sOnce you've done that, you can carry out the 'fine adjustment' at using the cable adjuster at the handlebars.
If that doesn't work, I would start to wonder whether the clutch itself is assembled correctly....