If you choose to pursue an upgrade, I would urge you to spend a little time researching Subs and reading some of the reviews here at AH.
After that, consider at least 2 Subs. These should be placed in the room where they perform at their acoustic best. As every room is different, I can't really tell you where that would be.
This is where taking measurements can come in handy. On the other hand, a somewhat crude yet effective technique can be the Subwoofer Crawl to help find places in the room where a Sub will perform at it's best.
Major things to consider is the Total Open Room Volume of the space you will put the subs in. Saying the "theater area" of your home is only 12x12' or some such is useless, especially if you are dividing a much larger space simply by how you use it. Subwoofers work on the entire room volume they are exposed to, thus if you have an open floor plan rather than an actual closed room, you must consider all of that space!
Now, this is not to say you have to "pressurize" all of that open space, however, you should take into consideration that all that space will impact how the Subwoofer(s) perform where you are using them.
Beyond that, are you in an apartment with shared walls or floor/ceiling? Are you willing to rearrange to place a Sub where it will perform at its acoustic best? How many seats are you trying to make sound good?
To that latter, if you are only worried about 1 or 2 seats, your needs may well be different than if you are trying to get 2-3 rows of seating covered.
One last tidbit is that you will control the gain on your Subs separately from running room correction which should help level match all of the Speakers including the Subs. Usually, you will set the gain so that they are outputting a test signal at a specific SPL which your room correction program will dictate (if it does at all). Once that is set, the room correction will hopefully then be able to match up all Speakers so the sound is uniform.
It helps if you can check that on your own using an SPL Meter or phone app.
In the end, this can be a complicated deep dive, or you can work a little to keep it simple.
The more Subwoofers you add, the more difficult it may well get. On your AVR, you will need to add additional gear to truly be able to control 4 Subs. Unless you are able to keep things perfectly symmetrical, that is... which will require a little bit of luck, too.
If you can answer the questions about total open room volume and consider where you are willing to place the Subs, then you should be able to easily move forward with a little more certainty about your choice!