WES assesses all your educational credentials that does not mean that you must attain bachelor degree before masters degree. If you only have bachelors degree you will get a WES certificate stating 4 years. If you have bachelors and masters degree you will get a WES certificate stating 4 years bachelor plus masters degree. If you only have masters degree then you will get a WES certificate showing only masters degree.
WES is not only for immigration, it is for assessing your degrees for future employment in Canada, applying to jobs and schools, and citing it in your CV in general. This is why it is important to include all your educational credentials.