The Oxford RSV notes that 4 Macc 10:4 does not occur in certain MSS: it goes on to say that this passage may be an interpolation. Furthermore, the passage may not have been written until sometime after Jesus' death (ca. 63 BCE-120 CE). Thus it may truly have no bearing on how we should understand Mt 10:28.
As for eternal torments: Neither Matthew nor Luke indicate that eternal suffering is being discussed by Jesus since Matthew writes that the soul can be "destroyed" by God "in Gehenna," while Luke notes that Jehovah "after he has killed," has the power to throw into Gehenna (Lk 12:4-5).
Furthermore, TDNT IX:646 observes:
"In Mt 10:28, however, the reference to God's power to destroy the YUXH and SWMA in Hades [Gehenna] is opposed to the idea of the immortality of the soul. VII, 1058, 15. For it is again apparent that man can be thought of only as a whole, both YUXH and SWMA. This view of man comes up against the undeniable fact that men are killed, e.g., in the persecution of the community. As Mk 8:35ff . . . already maintains, however, the YUXH, i.e., the true life of man as it is lived before God and in fellowship with God, is not affected by this. Only the SWMA (-----> VII, 1058, 15ff.)"
In harmony with other OT usages, Mt 10:28 could be using YUXH in the sense of life.
Additionally, John L. McKenzie (SJ) has some interesting observations regarding YUXH and Gehenna. He writes:
"Gehenna is also mentioned frequently in the rabbinical literature where it also appears as a pit of fire, a place of punishment for the wicked. In rabbinical literature, however, the eternal fire is not surely eternal punishment. The rabbis at times see the possibility of annihilation of the wicked or even of their release after a period of punishment" (McKenzie 300).
For a text that I think has a bearing on Mt 10:28 and the use of Gehenna there, see Isa 66:24.
McKenzie also adds:
"It [Gehenna] is a place where the wicked are destroyed body and soul, which perhaps echoes the idea of annihilation (Mt 10:28)."
He also contends that the "apocalyptic imagery" contained in certain NT passages should be taken for what it is, to wit, "imagery." The pictorial nature of "torments" should not be construed as "strictly literal theological affirmation" (300).
Gehenna is evidently neither a literal geographical place (in eschatological texts) nor an eternal locus of torture: Jesus seems to use the term in a figurative way. Gehenna appears to be representative of everlasting oblivion. NT Wright's advice is sagacious in this matter:
"It should of course be noted again that 'Gehenna' is the name of the smouldering rubbish-heap outside the south-west corner of Jerusalem . . . The extent to which it is used in the gospels metaphorically for an entirely
non-physical place of torment, and the extent to which, in its metaphorical use, it retains the sense of a physical conflagration such as might accompany the destruction of Jerusalem by enemy forces, ought not to be decided in advance of a full study of Jesus' meaning" (Jesus and the Victory of God, page 454-455).