Possible view #1, (and my favorite, which does not admit to the notion that the saved can become lost)
—There are quite a few places where different languages can 'legally' be translated two ways. In some languages, people simply talk a certain way, but when they read it, it somehow gets a different meaning. If I was to ask a believer if God can sin, they might scoff, and say, "What? That's impossible!", by which they might mean, "That is logically self-contradictory." Hebrews 6:18 says, "...it is impossible for God to lie...", in the same way, I think—it is not dealing with a question of God's power or ability. One can extrapolate it to be saying that God lying is a bogus concept—a self-contradictory notion.
So it is also with verse 4, "It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit..." and what follows is more of the same statement. It is saying that for someone regenerated to fall away is impossible, 'and here's why it is impossible'. For someone once born again to fall away is a self-contradictory notion. —my opinion, of course.
Possible view #2, (in contrast with #1, but still not admitting to the notion that the saved can become lost)
—There is, however, a sense of sharing in the benefits of employing the mechanics of "The Way", and I understand that there are OSAS theologians who read it that way; and, I must admit, there is a LOT of empirical evidence (ok, it is Anecdotal Evidence) that a person can "experience" even a sense of belonging to God, and a feeling that the Spirit of God has witnessed to their spirit that they are 'sons of God', and have fooled themselves. There are plenty passages employing severe language concerning the conduct of the born-again, to which not many of us have to gall to say we live up to it. Any of us can have lied to ourselves. These theologians would say that this is the person who is ultimately and surely condemned, because they have put Christ to shame, etc.
And Hebrews 10:26-31 would seem to affirm that view.