Your Marantz is rated at 100 Wpc at 8 ohms driving 2 channels. I'd ignore that 140W at 6 ohms rating because it's method of rating is not similar to the method used for the 100 W rating:
100 W + 100 W (8 Ω/ohms, 20 Hz – 20 kHz with 0.08 % T.H.D.)
140 W + 140 W (6 Ω/ohms, 1 kHz with 0.7 % T.H.D.)
As you said, Marantz says nothing about 4 ohms. Fortunately, your AVR does have preamp output jacks allowing you to add an external amp.
Dynaudio
rates the Emit 50 speakers as 4 ohm with an IEC power handling upper limit of 240 Watts. IEC ratings are for short-lived power peaks. In contrast, the 100W rating of your receiver is for sustained power, unlike the IEC rating.
So, it isn't clear if your receiver is or isn't up to driving the Emit 50 speakers. It probably will not overpower them, but it might not be stable with a 4 ohm load if at high volume.
A foreign language (Czech?) review of these speakers does show both a frequency vs. impedance (red trace) as well as a phase angle vs. impedance (black trace) curve.
View attachment 62908
Overall, the impedance doesn't look bad, it doesn't go lower than 4 ohms. See the red trace in the 80-300 Hz frequency range. But the phase angle does shift rapidly over that same range. The phase angle units, on the right side of the graph, make it hard to guess the range of angles over that frequency range. But I'd guess that it isn't small. Low impedance combined with a large shift in impedance phase angles, over the same frequency range, is a sure sign of speakers that are difficult to drive. So, it's possible that these speakers might be a difficult load for your moderately powered Marantz AVR to drive. But I can't say that for certain.
If you get those speakers, I'd try running them in stereo with your Marantz. But run them at high volume with some caution. See what you think about their sound.
Are you buying these speakers from a dealer, or online? What power amp does the dealer use to demonstrate them in the store? As always, be very cautious about believing what a dealer might say about this question. He will be only too glad to sell you a power amp with those Dynaudio speakers.
If you decide you want more power to drive these speaker, and if you're in the USA, I'd suggest looking into Buckeye amps. See these two 2-channel amps below. They're both very high quality class D amps. I've heard the more powerful NC502 amp in action, and I think it leaves nothing more to desire. The less expensive NC252 might be perfect for your needs.
Hypex NC252MP Amplifier, 2-channel, $575 for 250 W/channel
or
Hypex NC502MP Amplifier, 2-channel, $750 for 500 W/channel