Increase spells per day.

[Zelfin]

I have searched and not found an answer. So short of fame and the guild bonuses (not allowed by my GM), is there another way to recover lost spellcasting levels?

i.e. I take 1 level fighter, then 3 wizard. Is there a way to recover that 4th level of spellcasting? Not just increase the spell difficulty but gain spells per day also.

[Ray-gun]

Not that I know of. The closest is favored prestige class feat and prestigious caster feat. These combine to regain one lost caster level due to that prestige class. You could discuss with your GM if this would work for another class.

[Athaleon]

If you took the Fighter level for eventual entry into Eldritch Knight, you could gain the Martial Weapon Proficiency with Variant Multiclass Oracle instead (take the Skill At Arms revelation from the War mystery).