Sorry about the confusion. Yes there are only four summons per summoner, however as is also stated in the first paragraph, the first summon has the capability to evolve into what I am calling the 'fifth' summon, since it will undergo some mega changes in power, appearance and personality.

Lowest= 1 and 2
Middle= 3
High= 4
Highest = 5 (the evolved form of 1)

The very first summon should definitely be much weaker than the fourth summon. I was planning on the 1st summon being the very first one that our characters summon when they were young children.Meaning that the 1st summon is low level since a child cannot expect to summon a high level summon. 

This is where the whole 'words have power' thing comes into play. When a summoner calls the name of a being from the aether zone, that being is summoned forth and into the human world. And yes, the marks do come into play here, for without one even if a name is known the being cannot be summoned. The marks basically mean magic and so our characters are a tiny bit like witches, wizards and warlocks, but the only thing they can do is summon beings which are the ones who perform magic based on their type. 

The number of marks doesn't matter, characters can have as many and/or as few marks as desired, but the marks must relate to the type(Spirit, Air, Water etc.)

Attack = Things like Claw strike, kick, the usage of a weapon if the summon has one, etc
Defence = Things like shield, phase, block etc
Special = Things that have something to do with the summons type so say poisoning is a special ability used by nature type summons. Bindings could be a special ability used by earth type summons etc.

[b]I will post my characters CS soonish which will show an example of what I expect.[/b] If there are anymore questions, please ask away :)