Project for the Web lag and lead time

Hi,

Is there any way to create a dependency with lead time? For example, in construction i have a Slab Concrete Pour scheduled for a specific date and i know that i need to deliver the rebar for this slab 4 days earlier. So i would like to create a task for Rebar Delivery to be dependent on the Slab Pour Date, but 4 days earlier. 

In the construction world deliveries are usually based on the Pour date. This is a basic function in the Project Desktop program and i am dumbfounded that it either isn't available or obvious in Project for the Web.

Can anyone shed some light on this please?

Thank you!