In the 4E, a Wizard power known as Foe to Frog (Heroes of the Fey Wild) allows the wizard to change the target into a tiny beast.  My question is: can the polymorphed target retain its spell casting ability if it has the appropriate limbs for somatic movements and a mouth for vocal ability? An example: a necromancer turned into a frog. Ribbit ribbit (magic missle!). Interestingly enough, I don't consider a frog a beast, yet that is the example given.