Jeffrey, who said you need a 4-5" well for a mast step?
Also, going back a couple of posts, what do you mean by "anchoring my mast to the deck step" ?
And where does the question of "bury" arise in a mast step?
You need the mast well fastened at the heel so it can not rotate or lift, as advised by David who described a good detail, though not necessarily the only way to do it. A very experienced ocean sailer set out from these shores some years ago - but overlooked the need to secure the mast at the heel. Unfortunately the yacht inspector who signed it off and gave him clearance over-looked it too (as he told me). The boat was lost in rough seas in the vicinity of Great Barrier Island, before even reaching deep water, due to the mast lifting off the mast step and punching through the bilge.
You are not dealing with bending forces right down at the heel, only rotating and lifting/shifting laterally. This has nothing to do with "bury". You can drill and fasten at the heel (or David's better suggestion of a removeable pin together with a securely fastened housing or brackets for the pin) and it does make sense to allow for drainage if you are going to make some kind of socket. A shallow socket there in addition to th above, if you want to have "belts and braces" - provided that does not structurally compromise the keel or the mast step (as I think Ueli is reminding you). But the depth of the socket is almost irrelevant if the mast is properly fastened down and can not lift.
The place to secure the mast against lifting and rotating is the heel, not at the deck. Here, bending forces are the issue. You need a robust structure there to provide the support needed to keep the mast upright, and the mast should be free of welds, fastenings or bolt holes at that point. This is not based on ocean cruising experience, it is simple engineering. David has offered you some good advice based on both, and it has the additional merit of being simple and practical.
The distance between the deck structure and the heel is what is meant by "bury".