Hey Ed,
The issue is that WEBENCH checks the VinMin of the IC spec and it fails the initial check. Technically for BuckBoost designs, the VinMin is Vout + VinMin but we don't have a way knowing what Vout will be so we use the gross check using the IC's VinMin and VinMax.
To get around this problem, use VinMin = 4.5V, it should come up with two parts you can use: TPS84259 and LM5008. Let me know if you have more questions.