Can I use up to four VoiceSolo monitors on one 4 ohm amp channel?.
You can if you reduce the level of the volume control by 2 clicks from full on each speaker. This brings the impedance seen by the amp to 3.5 ohms and of each monitor to 14 ohms with a slight reduction in volume level.
Unless the amp is running full volume, most modern amps should be able to run at 3.5 ohms.
The attached impedance chart shows the impedances at all volume control settings.