I don't have the figures for Category A & B aircraft on my Jepp chart, but if the back of the aerodrome chart has your figures "for filing as an alternate" (actual or forecast QNH [100' diff]) 1964' and 4.4 km vis then, if either of the forecast cloud height* or visibility is less (and both are in your example) then yes you will require to plan an alternate.
And yes if using Canberra as your alternate, its forecast must be better than this as well.
S-I = Straight In Landing. Presume it means instead of Circling to Land (off the ILS) which has a higher minima.