4bbl vaccum vs mechanical secondary, what does it mean?

Mechanical secondaries open up when the throttle reaches a certain point. Usually not long after the throttle is pressed.
Vacuuum secondaries only open up when the engine vacuum reaches a certain level.
There are a lot of variables to determine which is better for your setup such as gears, transmission, vehical weight,...etc.
Usually, lighter cars benefit better with mechanicals and heavier cars are better off with vacuum but like I said, it all depends on engine setup.

Myself, I would go with the vacuum carb. They are more "forgiving" than mechanicals.
If you had more mods done then I would say mechanicals.
Compression ratio also plays a part too.
Vacuum carbs will only open the secondaries when the engine needs it.
Mechanicals open the secondaries if youe engine needs more gas or not....in a sense. More tuning is required with mechanical carbs.
I think you will be happier with a vacuum carb. IMO