I have just tested the 3000km old injectors I had in the car. I have 2 good ones out of 6. I think I understand now why I had 4 cylinders more rich than the other 2. Those 2 had good injectors.
I will now go through the steps:
1. Air blow after carb cleaner
2. Carb cleaner soaking overnight
3. Ultrasonic cleaning
4. Air blow
5. Carb cleaner flush on the tester
Test and if not good
6. Backflush air and carb cleaner
I think that backflush is dangerous as you have to open the injector (I made a tool as on the picture) and this is when you can damage it completely. On the other hand, as I understand how an injector is built, back flush is only to remove dirt from above the filter mesh, but this you probably can also do via air blow. If injector is dirty/stuck because of dirt in the end chamber, back flush will not help.
I think odds for success are small, I will end up buying 4 new injectors, hoping they are ok.