I bought a quadplex with under-market rents, but it was priced accordingly. Naturally, once we started to raise rent, all hell broke loose. There were extra periods of vacancies. Spats with the current tenants. Unfortunately, they were all long-term, so we didn't have the option of simply waiting them out. I tried my best to be upfront with them and give them extended windows to plan ahead, but it's just impossible to go through a process like that without things getting emotional and messy. You'll need to take that strongly into consideration.
And yes, quadplexes are the sweet spot because they qualify for residential loans.