how does tier 4 works

i am totaly a noob

for example i completed till tier 3 and it says in tier 4 all jacks -1 point cost
does this means the next helljack -1 point cost or all jacks right now minus 1 point

All jacks.

Basically once you reach tier 4, all your currently listed jacks will get the discount, so you get surplus of points to buy another unit.

does things light er ripjaw defiler helldiver deathripper counted as helljack ????

If they look like a chicken, they're bonejacks. Otherwise, they're helljacks. When in doubt, refer to the subtitle. It normally reads "Cryx Bonejack" or "Cryx Helljack" or somesuch.

ooo means if i have 5 slayer then will become automatically 4 if the tier list say -1 to helljack

you will also need to fufill tier 1 to tier 3 requirements as well.