How soon can we start planting annuals in zone 4b
You will need to wait until the danger of frost has passed.
As a zone 4b garden myself, I know that this is a date that changes and you need to monitor the weather, sometimes even into mid May we can see a frost.
May 15 is the safe date. If you are potting up annuals you can often do so earlier but may need to cover or move indoors if our weather takes a dip.