The use of the nomen sacrum IHS at Heb 4.8 suggests that P46 might be interpreting this verse in terms of "Jesus" rather than "Joshua" (an option discussed in the commentaries, e.g. Ellingworth). It seems that readers of P46 would have taken it this way (cf. earlier in Hebrews 2.9; 3.1; but also present as a three letter nomen sacrum throughout Romans, which immediately precedes Hebrews in P46), by way of contrast "Moses" is never contracted in P46. This view has the advantage of not needing to introduce a new subject for 4.8b.
I suppose the alternative is to think that the scribe of P46 was well acquainted with LXX MSS which already use nomina sacra for rendering IHSOUS = Joshua in OT texts. I'm not sure that can be documented for the period of P46.
Up-date: See here for a late second century papyrus codex of Joshua featuring three letter nomina sacra IHS for Joshua (right hand page, line twelve).
