Formula - Sum of Multiple Group Header data

Hi,

I'm looking to create a formula in Group 1 Footer  that sums data from the Group 3 Header and Group 4 Header.  I've done variations of this, but always in some way a Sum of a Sum, where the Header level is already in the picklist.

Having a devil of a time getting the syntax right.

Right now I have:

sum({Header 3},{Field}) + sum({Header 4},{Field})

What am I doing wrong?  How specifically does Header 3 need to be referenced?

Thanks!

Matt