I'm still using the 1st release of the SR4A pdf so hopefully they corrected this line, but on page 183 under "Area Spells" it has this printed:

"Dice expended to change the radius of effect cannot be used in any related test, such as resisting Drain for that spell."

Isn't that from 1st-3rd edition only or can you use spellcasting dice for drain in 4th too?

No. But the statement itself is rather obsolete. It's just there to make it clear that you can't relocate dice from one pool to another.

You where never able to withhold dice from Spellcasting to use on your Drain Resistance in SR4, with the single exception of Spellcasting/Summoning Foci - which was removed prior to SR4A.

So... no you cannot use use dice from Spellcasting to assist with Drain? If so, they really need to pull that line from the book.
Ah, thanks Muspellsheimr. You answered while I was posting.

Not overly concerned, just looking to have the data.

Where was it removed? So I can show my players before they make the next campaign's characters. Previously, they liked foci. They may not now.

IIRC it's mentioned in the errata.