@toasca by official server, do you mean its a retail server where it is pay to play?
anyway, your example is possible, getting 3-4 souls from a x4 mob...
it also should be okay getting this number from this mob, because the mob is x4 stronger, in terms of Hp, than the regular mob...
however, do you really think that gaining 10 souls from a regular mob is fair?
that this will not make the game imbalanced?

also, what will you call the lost xp when the kamael absorbs a soul?
as the description says, it needs #### xp to absorb a soul...
meaning the ### is the amount of xp that will be "consumed"...
or if you dont like to call it "consumed", it can be called "taken away"...
either way, they mean the same thing, you will not get the 100% of the xp given by the mobs...