How do I up AC bonus on homebrew content

I've created a shield for a campaign. I want it to be +4 AC , but Beyond seems to only allow +2... can I change it to +4?

Add a Modifier to the item. Set "Modifier Type" to "Bonus", set "Modifier Subtype" to "Armor Class", and set "Fixed Value" to the extra AC you want it to provide. If you want the total AC bonus from the shield to be +4, set Fixed Value to 2 (because you want 2 more than a regular shield's +2).

That said, if you want the total AC bonus from the shield to be +4 and it doesn't need any other special properties, then you don't need to homebrew anything; just use the existing Shield, +2.