Reading between the lines, it sounds like you really love music and you're trying to figure out the best way to "plow with the horses you've got". I respect that.
If I understand correctly, your stacked Advents are currently wired in series for a 16-ohm load, and you want to drive a pair of 4-ohm Ohm Model L speakers also. Is that correct?
16 ohms in parallel with 4 ohms = 3.2 ohms, but a "4 ohm speaker" may have an impedance curve that dips down well below 4 ohms. If you have a fairly beefy receiver that is rated for 4 ohms (very few are rated for less than that), AND you are careful not to push it to the point where the output stage overheats or audibly distorts, you should be fine. But approach with caution.
The power distribution to the speakers will not be so good: Roughly 80% of your wattage will go to the Ohms, and roughly 20% to the Advents, so the Advents really won't be contributing all that much. Just so you know.
Let's look at the other possibilities. If you hook up the Ohms in series with the Advents for a 20-ohm load, their respective impedance curves will modulate one another's frequency response, which is a roll of the dice. The power distribution will be reversed (80% shared by the two Advents and 20% to the Ohms), which is better, but now your receiver will voltage-limit and clip at less than half its rated power into that 20-ohm load.
As long as we're rolling the sonic dice, you might want to consider this third possible configuration: Wire the stacked Advents in parallel for a 4-ohm load, and then wire one of the Ohms in series with each paralleled pair of Advents. The net result will be an 8-ohm load, so your receiver will be happy, and the power distribution will be 50% to the Ohms and 50% shared by stack of Advents. I make no promises about how this will sound, but it might sound good, and imo this is the best configuration from the standpoint of your receiver's health, your speakers' health, and getting as much SPL as you safely can.
May the audio gods smile on you in your quest.