Whether fixed terms are constitutional or not is sort of an open question - but it's already been done in BC, and the next election in Ontario and in N&L will be held on a fixed date. In Ontario it will be held next year, on my birthday no less - which is a fine date as far as I'm concerned - and in Newfoundland it'll be held a few weeks later.
Fixed election dates, as they've been implemented so far, do not preclude early elections in the event of a no confidence vote. If that were to happen, an election would be called right away, and then the next one would be set for four years from that date (give or take a day or two - most provinces have rules that elections must be held on the same day of the week each time - here in Nova Scotia, it's always on a Tuesday, for example).