Probably they had some contractual commitments with TSMC and had to use up their N3B capacity somehow. But as soon as N3E became available it’s a much better process overall.

Ramping up production on a new die also takes time. The lower volume and requirements of the M4 as used in the iPad can give them time to mature the line for the Macs.

So far, I haven’t seen any comparison between the iPad M4 and the computer M3. Everything was essentially compared to the last iPad chip, the M2.

Your laptop M3 chip is still probably more powerful than this. The laptop M4 will be faster, but not groundbreaking faster.