We can only speculate why Larian chose d4. My guess is that it was intended to liven up combat; your party is more likely to get the same rolls and you switch order to can set up combos instead of 1 by 1 as it is in tabletop. If this was the case, judging from reddit it did not succeed. Many people don't even know you you can do this.
I believe d20 would be the better path, and I use a mod that does just that. Alert is still amazing, but it is more appropriately balanced.
