Grinding AI3 and winning 100% of the time in ~90 seconds is much, much better than grinding AI4 and winning 50% of the time in ~180 seconds.
Please note that I made all of these numbers up out of nowhere in order to support my hypothesis, results may vary. But yeah it's generally supported by the community that grinding AI3 is more profitable than AI4, because games are faster, winrates are higher, and that = more electrum over time than AI4, even considering the few times that you'll get upped cards from AI4.
Also, playing more games more quickly will also give you more card spins, which = much more cards and/or electrum, plus you occasionally get rares from AI3.
Were I you, I'd be grinding AI3 until I could build a deck capable of grinding silver reliably, which is much more profitable than AI3 OR AI4.