John, XM was quoted over and over as saying that it is XM-5 that has the enhanced capabilities to broadcast over the Sirius spectrum.
XM-4 is essentially identical to XM-3.
Here are the Technical Spechs for XM3 and XM4 that are in file with the FCC. You'll see that they are identical, except that XM-4 has larger propellant tanks and additional thrusters -- but that's it.
http://us.f13.yahoofs.com/bc/460ae62...u7hoJBAL7CGgc2
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