Extra Slot( Tome and Blood: A Guidebook to Wizards and Sorcerers, p. 40)
You can cast an extra spell.
Spellcaster Level 4,
You gain one extra spell slot in your daily allotment. This extra slot can be at any level up to one level lower than the highest-level spell you can cast. For example, a 4th-level sorcerer gains either an extra 0-level or 1st-level slot, allowing him to cast any known spell of the chosen level one more time each day. A 4th-level wizard can prepare any extra 0-level or 1st-level spell she knows. Once selected, the extra spell slot never migrates up or down in level.
You can gain this feat multiple times. Each time, you gain an extra spell slot at any level up to one level lower than the highest-level spell you can cast.