I'm playing myself with two Avacyn Restored decks from a 6-booster sealed prerelease tournament, and wonder whether a 6/4 creature with trample blocked by a 4/1 creature would still allow damage through.
Combat damage is all dealt simultaneously (in two steps, the first of which is exclusive to creatures which have First Strike or Double Strike) - assuming the attacker and blocker both do not have First strike or Double strike, the four damage from the blocking 4/1 is dealt to the 6/4 trampler at the same time that the 6/4 with Trample deals one to the blocker and five to that creature's controller (assuming there are no other blocking creatures).
Thus, if you are at five or less life, and have a 4/1 without First Strike or Double Strike blocking a 6/4 with Trample, lethal damage will be assigned to you (and dealt, unless it is prevented somehow).
If your 4/1 blocker does have First Strike or Double Strike (and the 6/4 Trample does not), your blocker's combat damage will be dealt in a combat damage step earlier than the 6/4, and if the 6/4 is dealt lethal damage (which it would be, assuming no combat tricks), the 6/4 would be destroyed and would not be present for the second combat damage step, so you would avoid damage altogether.
See the following from the 5/1 comprehensive rules: