You are correct on the 2.4 ohms array impedance if you start with nominal 4 ohms drivers. If the drivers were 8 ohms nominal, then the 3/2/2/3 feed will yield an array impedance of 4.8 ohms.

Now you can still achieve the same power ratio (1.5 to 1 in the case of the 3/2/2/3 connection) if you do a 6/4 connection with your 4 ohms drivers. What you do is to connect drivers 1, 2, 3, 8, 9, and 10 in series for the 6 and then connect 4, 5, 6, and 7 together to get the 4. Then parallel the two sets of seriesed drivers. The array impedance would be 9.6 ohms which may be more amplifier friendly if you have an amp that has problems with less than 4 ohms loads.

Jim