Can I have four 120V circuits on a Quadplex breaker?
It's vitally important you know what a MWBC is and how to recognize one, when you are doing things with existing circuits and quads/2-poles.  Placing both legs of an MWBC on the same pole Is Very Bad and can start a house fire.
That said, yes, 2-pole breakers in general can be used for two independent circuits.
However, in renovations you typically need to build to modern Code.  It's very common for people to do a reno and go &quot;Aw, I'll just re-task the original wires that came here&quot;.  Nuh-uh.  Your kitchen reno needs to comply with modern Code, and that includes wiring, and that includes AFCI/GFCI breakers as required.   And where AFCI/GFCI breakers are required, that precludes use of double-stuff breakers.  Which is going to be a problem in a tight panel.   Code says &quot;too bad; upsize or subpanel&quot;.
And yeah, that may be a good idea.  If you can find a suitable mounting point (e.g. facing a pathway so that nobody ever blocks the working space), adding a subpanel to the kitchen reno makes a lot of sense.  A well-powered kitchen needs 8 circuits or so. That lets you remove all existing kitchen circuits from the overstuffed main panel and substitute with just 1 2-pole.
Now, we strongly recommend wildly oversizing any panel; so if you just heard &quot;8-space panel&quot; what we meant you to hear was &quot;30-space panel&quot;.  Spaces are dirt cheap when you're buying the panel.  And they save you a fortune later when you need to add spaces.
Not lightly do I spend your money, but please - spend $60 extra on another 20-30 spaces. It's the best investment you can make in electrical, bar none.