If you only have four days, and one of those is reserved for Niagara, then you really don't have time to go elsewhere. We recommend a minimum of 3-4 days for Toronto and a day for Niagara.
You need to start researching your Toronto stay - check the Top Questions (including trip reports) at the side of this page and the green info tabs at top of page. Also - if you want suggestions, please give us some idea of your interests- culture, foodies, sports, nature, etc. Note the beginning of April is still the end of winter here and some attractions, both in Niagara and the city, may not be available.