Patron Spells: At 1st level, a pact wizard must select a patron. This functions like the witch class ability of the same name, except the pact wizard automatically adds his patron’s spells to his spellbook instead of to his familiar. In addition, the pact wizard can expend any prepared spell that isn’t a spell prepared using the additional spell slot the wizard receives from his arcane school in order to spontaneously cast one of his patron’s spells of the same level or lower.
So if I multiclass a Pact Wizard, I'll only get the patrol spells of the my actual wizard level, because the Witch rules on Patron Spells specifically says that you only get the new spells known at the listed Witch level. Fair enough.
My question though, is does he get the spontaneous casting for the entire patron spell list, assuming he learns it normally?
It's sufficiently ambiguous that I could see a GM ruling against it, but technically speaking it appears to work. You can take a 1 level dip of Pact Wizard and, provided you can cast the patron spells with your other spells, gain the ability to spontaneously converted prepared slots into those patron spells. Personally I don't think the loss of caster progression is worth it, not when there are feats that duplicate this effect.
I was already making an Arcane trickster when I noticed this;he's only losing one level of casting regardless, and spontaneous blast spells are a bit advantage to have one he hits the capstone.