That would be impossible. The Altus are explicitely a mage class. How will we be able to play as warriors or rogues in DA4 if we are Altus?
Sure, maybe we'll have Altus as a background for mages, but it can't be the only possible background unless Bioware suddenly changes the lore. Frankly, having a slave, a Tevinter elf, a dwarf from Abassadoria or a Tal Vashoth as the PC in a Tevinter setting would make more sense.
I think there is a bit of confusion about the phrases based specifically on what Dorian says:
If you're not a magister, then what are you called? No special title?
I'm an altus, which is almost as good as a magister, depending on who you ask.
I've never heard of an "altus."
Upper class. Those families who trace descent from the Dreamers, the first prophets of the Old Gods. If you're a mage and you're not altus, then you're laetan. Lower class. If you're not a mage at all, you're soporati. That's "everyone else." We do love our fancy words.
The phrasing, "If you're a mage and you're not altus," does seem to suggest that both altus and laetan are exclusively mage class.
BUT I don't find that this is entirely logical simply because genetics, no matter how much you plan out, are still a crapshoot. It's all about percentages and there will always be some chance, no matter how low, that an altus family has a non-mage child. So in that case, the non-mage should still be considered altus because of the family they were born into.
And as The Baconer pointed out, a mage child born to soporati enables the family to be elevated to laetan, which would include all of the other non-mages in the family.
However, there is one word that I've never been able to determine the meaning of.
In Dorian's WoT Vol. 2 entry when it talks about his friend whose house he was kidnapped from, it refers to that guy's family as vulgati. I haven't seen that anywhere else. For some reason I'm leading toward a nouveau riche type. Perhaps his father is a soporati business man who hit it big and is living large and mingling with altus society, but would be looked down on because he lacks the lineage and mage status. This makes the most sense to me, considering the source and tone of that entry, but I don't know for sure that that is the case.
Back to the topic of DA4 origins... If the above assumption is correct, then we could have the warrior or rogue PC be from an altus family. It would be interesting to see how that is portrayed. I'd imagine that there might be somewhat of a negative attitude regarding such children. But they have to be educated. If they don't have private tutors, then there might be special schools for the non-mage altus children since they wouldn't go to Circles. OR if they do go to the Circles, they would basically be treated like the "short bus" kids, and might even stick together as a result of that.
There are all sorts of interesting things they could do with the non-mage altus PC.