A fair share of languages use singular nouns after numerals - in e.g. Turkish, you say 'two man', not 'two men'. In part this is a reduction of redundancy, but on the other hand, redundancy can be a feature rather than a bug.
Now, let's consider a language that operates like Turkish on this count, but has an extra quirk: many determiners' stems also encode number, so e.g. the singular 'this' and the plural 'these' do not share a stem. However, both also use a full set of case congruence markers that encode both number and case.
For 'these.acc four.acc dog.acc', "these" would thus have a plural stem with a singular accusative suffix on it.
An obvious suggestion for a situation where the opposite could happen - singular stems with plural morphology - could be when the speaker wants to imply some kind of collective. Thus, collectives would be morphologically plural, and only marked whenever there are determiners involved.