PHEVs would get at least a $4K credit but most won't qualify for the additional $3.5K because that requires at least a 40 kWhr battery and no more than a 2.5 gallon gas tank. However, I think they could qualify for a bigger credit if they have a US made battery which adds $500 and/or are made in the US with union labor which would add another $4.5K.
One example would be the Rav4 Prime which would only qualify for a refundable $4K credit (car is made in Japan but I'm not sure where the battery is made) instead of the nonrefundable $7.5K it currently qualifies for. So if someone wants a Rav4 Prime it would probably be best to purchase a new 2021 or 2022 model this year (unless they don't owe enough taxes in 2021 to cover the $7.5K credit).
If the Build Back Better Act does not pass, then I think the current 2021 tax credits would remain until the maximum number of cars sold is exceeded.
By the way, here is the latest text of the BBB Act (as of Nov 3rd):
Build Back Better Act 11/3/21
The EV credit text begins on page 1480. Note that line 9 of that page says it will be a refundable credit.