The zero index funds have slightly more tracking error than the vanguard funds. Also if you are in taxable I would be worried about being locked in if fidelity

The zero index funds have slightly more tracking error than the vanguard funds. Also if you are in taxable I would be worried about being locked in if fidelity start charging fees on them. I think vanguard is a safer bet to keep fees low long term. And ultimately 4bps is irrelevant.