Sure can. 300pts for a naked slann (usually 355-360pts if BSB + sig disc/loremaster). That's a lot of points.
You can easily have an oldblood (doesn't have to be on carno) as your general and a couple of priests (scroll + cube). You can either go super low on magic with 2 x level 1, maybe couple it with a bastiladon or two or get 2 x level 2.
Since you need a BSB you can either get a scar vet or go cheap and just get a chief, with some basic defensive items and a BSB.
Personally I'm going to try out:
OB on CO w/GW, armour of destiny and dawnstone.
Tetto'Eko.
Beast Priest w/scroll.
Chief w/BSB, dragonhelm, LA and shield.
That's a total of 605pts.
Compare that to:
Slann w/high magic loremaster and BSB.
OB on CO w/GW, armour of destiny and dawnstone.
Beast priest w/scroll
Total of 701pts (which I usually take).
I've saved myself roughly 100pts and have other spells + special rules at my disposal.
Alternatively you can also get:
OB on CO w/GW, armour of destiny and dawnstone.
Level 2 Heaven Priest w/ cube.
Level 2 Beast Priest w/scroll.
Chief w/BSB, dragonhelm, LA and shield.
which costs 595pts. You can save an additional 70pts if you decide to go really low on your magic phase. Usually though I suppose one would do that because they're going with a carnosaur or two which tends to be quite an expensive solution.
I don't think we need a level 4 mage. It's neat alright, but not absolutely needed. Mostly though I think it's a matter of your skills and knowledge. A level 4 mage cannot dispel something nasty casted with irresistble force anyway so if you bring a couple of level 2 mages (tetto or 2 regular priests) you'll just have to know which spells to let to go and which you need to dispel.
The good thing with Tetto is that he brings a lot of good spells with him, coupled with a beast priest (wyssans) and you have 8 IMO good spells. The damage spells from Heaven are S5/S6 and while they cannot remove entire hordes like nobody's business they can certainly hurt a lot of good stuff. So in other words even though your opponent might have an advantage in form of a level 4 caster you can still do some seriously good stuff.
If you go with Tetto just be careful about using too many dice. You HAVE to reroll every dice of 1 you roll, so you can easily get yourself into an unfortunate situation of IF.