4 Slann total (Kroak + 3 regular Slann) would be 13 spells base; 4 for Kroak and 3 for each regular Slann. If you sacrificed each Slann's first spell, for 4D3 additional summoning, that's still 9 spells. Common spells are 1 Celestial Equilibrium, 1 Comet's Call, 3 Celestial Deliverance, and 1 Stellar Tempest (provided the last two spells are in range). That leaves 3 spells left for... Arcane Bolt? Mystic Shield? Another Seraphon spell?
To maximize the potential, you would need 1 Astrolith Bearer (or maybe even 2) to increase spell ranges and/or an Oracle on Troglodon to increase spell ranges through it. Then you need to consider Saurus Guard to protect some, if not all, of the Slann; definitely Kroak and your Slann general.
Your summoning would be 6D3 per round (1D3 for General being on the table, 1D3 for one or more Astroliths, and 1D3 for each Slann if they sacrifice their first spell). That's 6 minimum, 12-15 average, and 18 max. That would be one definite unit of 10 Skinks (two units with an average roll, or three units with a max roll) each round. An average to max summoning roll opens up a few more options each round; from a Salamander, or 10 Saurus Warriors, or 5 Saurus Knights to name a few of the units. Naturally, you could risk banking them to get something bigger.
That's a long way of saying I think 4 total Slann is a lot, and probably too many. I think you'll end up wasting casts on your hero phase. Plus, you lose a decent amount of the flexibility that Seraphon can bring with so many points tied up in Slann, and the units to boost/protect them.
Although Slann have some powerful board-wide features, you don't need only Slann to have a strong magic army. There are Skink Starseers, Skink Starpriests, and Skink Oracles on Troglodons as well. Specifically, Starseers and Starpriests have access to good spells on their warscrolls (let alone access to the Skink spell list) and their warscroll abilities are VERY useful to various units. Additionally, there is the Arcane Tome artifact to make any hero a wizard. That artifact on something like an Engine of the Gods, Skink Chief on Stegadon, or even a Carnosaur can make for a scary wizard.
All that said, if you just want to try a 4 Slann list, then go for it. I'm sure it will be a casting nightmare for your opponent. You could even try to bring some Endless Spells, if you can fit them into the list?
I hope this helps.
EDIT: One other thing. "Magic" also doesn't just mean wizards. Seraphon have Skink Priests and the Engine of the Gods which are both priests and can each chant a prayer in the hero phase. This can't be dispelled and the benefits are strong. Just the basic prayers of Heal or Curse can be very useful.
But having a bigger variety of units is also always a bonus, so I'll look around for the other ones you mentioned. 
