Not all properties need to be under the same LLC. The LLC is about asset protection. Look at the Katy Perry lawsuit. Do people think she doesn't have her own assets protected so she doesn't have to pay a dime from that lawsuit? Of course she does. Smart people protect their assets.
One issue about using multipe banking accounts is screwing things up where you use a bank account for the wrong LLC.
Let's imagine you are purchasing 4-plex for an investment and will be renting it out for profit. When setting up your entity, make sure it is structured properly to hold the title of the real estate. When you set up your LLC be sure that is it holding the title of your 4-plex. This structure also helps protect all your personal assets. For example, if a tenant of that 4-plex sues for falling on the property and wins the court case, they are not able to acquire all your personal assets like your bank account, the equity in your home, and all your other assets because the 4-plex is owned by the LLC. They are only able to access the assets in the LLC.
Personally I know some fellow rich people like me that purchased their homes under an LLC and rent it out to themselves.