When there are multiple iPhones in a household and you want to upgrade them all from a single iOS download via multiple Macs, it can be a bit of a challenge. I have previously blogged about how to do this. This post contains the link to the latest version (4.3.5) of iOS for the iPhone 4 which deals with a certificate validation security vulnerability.
iOS 4.3.5 Release Notes
Fixes a security vulnerability with certificate validation.
The iOS 4.3.5 firmware update file (.ipsw) is available at:
Update each iPhone as per my previous post.