This panel (Square D Hom612L100) has 4 slots. Is it possible to make one of them serve as the main breaker?
You can do this, but it burns up two panel spaces in the process
While the HOM612L100 is a Main Lug Only panel and thus cannot accept a QOM main frame breaker, it can be fitted with a main breaker. How? By backfeeding the panel's busses via a two-pole Homeline branch breaker that is held down with a HOM1RK hold-down kit, as required by NEC 408.36. As a result, the incoming hot wires go onto the lugs of this held-down breaker, and the main lugs can be left unused, or used as feed-through lugs in case expansion is called for.
Note that this requires the main breaker to be a regular (or triplex/quadruplex) Homeline branch breaker, not a GFCI or AFCI (they cannot be backfed as it would damage their electronics), and also costs two panel spaces in what is already a tiny panel; depending on your application, moving up to say a 12-space panel that accepts a main frame breaker may be a better solution as a result.
If it's the same as what I'm seeing on Home Depot's website, I'm going to say no.
A main would serve to cut power to the hot busses in the panel. As I can't actually see what the inner parts of the panel look like, I'm inclined to say that it's probably just an always-on sort of thing. If you want a main for it, get a service disconnect to go between that panel and the utility feed.