Making these spells available to every class through potions solves several problems:
-Improved Invisibility in a bottle is a costly but viable alternative to having an ideal party composition.
-Divine Power can let a flimsy mage save themselves, or enhance the damage output of Dex builds against bosses.
-Sludge drinks already exist; this normalizes their presence.
-Death Ward allows crusaders to address a very common threat when facing Undead.
300gp+ is a good starting point.
I would also suggest that, to balance this, 4th circle potions add a significant dose of Seam Sickness.
Ideally, all player-crafted potions should add a degree of seam sickness relative to their GP cost, whether they came from seams/wizards or not; BUT 4th circle potions specifically ought to have a prohibitive cost beyond GP.
No. :)