We can see from Apple's 2024Q4 10-Q that "Revenue" is described as "Net sales", and that Q4 had $8 billion for the IPad, so about $32 billion for the year - comparable to the prediction of "$30 billion ad business by 2026".

Total net sales in Q4 was $124 billion of which $26 billion was services (where ads and Google income are placed).

The $4 billion was from a few years ago. The actual number doesn't seem to be published, but the numbers seem more like 7-8 billion, which is about 5% of revenue, not less than 1%.