Do yourself a favor and play Etrian Odyssey IV first, it's more in the spirit of the series than Untold (unless you plan on playing classic ode for Untold), and the gameplay mechanics are arguably better. From what I've been told, there are two Etrian Odyssey teams, the mainline team (which also handled development of Persona Q), and the spinoff team which did Untold 1 and 2.
Etrian Odyssey IV is very accessible but has plenty of content and doesn't force story down your throat too much, which is really what made me fall in love with the franchise. The cyclic nature of the game, especially its resource management and mapping, doesn't need all this excess fluff. That's not to say EOIV doesn't have a story, though- what is there is pretty good, but not as intrusive. The Untold games add an excess layer that is deeper if only because it revolves around characters instead of the mission of exploring Yggdrasil. EOIV's classes are also more expanse (featuring multiple different roles instead of a linear skill progression), and it has overworld exploration, which is an additional and totally awesome wrinkle that is absent in the Untold entries.
I recommend EOIV first just to give you the groundwork for being excited or interested in EOV, which will likely take cues from that installment more than the Untold series. However, if you play EOIV and you want more in terms of character, you can move to Untold. That's the path I took. I can see wanting to start with Untold first, though, as you can ease yourself into the style of gameplay and then be blown away by EOIV, but I worry that you might end up disliking Untold and skipping the greatness that is EOIV.
I dunno, maybe I'm over-analyzing it, because I wasn't fond of Untold all that much after playing IV. Still, I think IV is a high point and deserves to be looked at when you find time.