Do skill stacks guarantee a skill level up?

The title says it all . . . heh. Anyways, I understand the basic concept of leveling skills. You sacrifice a monster with the same skill to level it up, and it's less likely to level up your skill the higher level it is. The question that's bugging me though is how most people sell stacks with a level of four. I've bought some already and I've already leveled up some of my monsters, but I'm afraid of sacrificing some more because I'm afraid it would just end up in a power-up. When you sacrifice a monster with a skill of 4, is there a 100% chance you would get a skill level up? If not, when's the best level to start sacrificing skills of 5+?

Thanks!

there are no guarantees except when you use a sos/+

the reason we use stacks at (4) is because (4) give a reasonably good chance of skill-ing up monsters whose skills are 4 or higher already.


you will find different opinions but usually i lvl from (1) to (4) using singles, then (4) to (20) using stacks (some will fail but generally not too many)

Thanks guys! It's a huge help, the problem's been bugging me!