Your English is better than mine - I have no trouble understanding your posts.
Then there is British English, American English, Australian English, etc. etc..
Rarely will a 4 speed transmission jump out of gear without some visual clues on the related parts. Look at the parts closely. Look for abnormal or heavy wear patterns and signs of over heating.
Then you need to look beyond the camplate, shift forks and gears. As Richard said above, some shifting problems are related to problems with the shifting mechanism in the outer transmission cover. It is important for the two springs that centralize the shift lever to be balanced, or equal tension. These springs lead a tough life as they are often exposed to a lot of condensed water.
I also see from your flickr account you have bought a lot of new parts for this engine. If you have bought new index plungers (little fingers) for the shift quadrant. Current production has created a problem... There are people making five speed plungers that are not marked (identifying groove around diameter) as they were at the factory. A lot of parts vendors are unaware of this and get the 4 and 5 speed ones mixed up.