For simultaneous dual-band APs, the AirPort Utility displays Low/Medium/High instead of numeric rates in the multicast rate pop-up menu because it means different things in the different bands.
Here's what those settings mean, per band:
Low: 2 mbps in 2.4GHz, 6 mbps in 5GHz
Medium: 5.5 mbps in 2.4GHz, 12 mbps in 5GHz
High: 11 mbps in 2.4GHz, 24 mbps in 5GHz
Note that the AirPort Extreme's default "Automatic" radio mode supports 802.11b clients in 2.4GHz, so the multicast rates for that band in this table are 802.11b CCK and 802.11-1997 DSSS D[B|Q]PSK rates. In contrast, 802.11 in 5GHz started with 802.11a OFDM rates, so there are no 1/2/5.5/11 rates in that band.
If you change your radio mode on the 2.4GHz radio to G-only or N-only, the 2.4GHz band "Low/Med/High" multicast rates match the 5GHz values in the chart above. Note that to see some of these options, you'll need to hold option on the Mac or alt on Windows while clicking the Radio Mode pop-up menu. Also note that "N-only" requires clients to be N-capable to join, but it doesn't force clients to use only N-style MCS-based rates; it still enables the A/G OFDM rates so that clients can choose those rates for possibly greater range or power savings.
Contrary to what you said in your Question, there was never an option for a 4mbps multicast rate because there is no such thing as a 4mbps data rate in 802.11, a, b, g or n.