I was wondering whether Wilfred ever realises that he is the one to fulfill the prophecy that ultimately kills Tennant. In the episode, Wilf says that he thinks the master is the one to fulfill, but we found out it is in fact him, but does he realise it?
Wilf's dialogue that accompanies his knocking suggests that he does not understand, at least initially, the gravity of what is happening:
WILF: They gone, then? Yeah, good-o. If you could let me out?
The prophecy said "he will knock four times", but actually Wilf did not knock only four times - he knocked more times than that (I think it was 8 or 12), so he may not have realised it for that reason too. Of course, he did knock in a rhythm of 4 (like the heartbeat of a Timelord) which is why we connect it to the prophecy.
The Doctor goes on to explain to Wilf "You were always this. Waiting for me all this time" (meaning that Wilf was the one to knock four times), but overall, the Doctor's speech is fairly cryptic and Wilf's stand of "just leave me" (to die) remains the same before and after The Doctor's explanation. It seems to me that, even after the explanation, Wilf does not realise his knocking fulfilled the prophecy.
Having said that, it may not be strictly correct to say Wilf uniquely fulfilled the prophecy and that the Master did not. The whole event that led up to this was engineered by the Timelords and The Master and the rhythm of 4 in The Master's head had permeated all through space and time. It is such a major thing to be dismissed in place of Wilf's knocking on a door. Interestingly in religion, prophecies are often said to have more than one fulfillment - a major fulfillment, and a minor fulfillment. This two-part episode began in a church where the TARDIS appeared in the stained glass, and the inference was that some aspects of real earth religion were perhaps based on The Doctor. It is possible the intention is that both The Master and Wilf fulfilled the prophecy in different ways. Alternatively, the intended narrative may be that prophecy is inescapable and that, if the Master did not fulfill it by killing The Doctor, then it would be fulfilled some other way.