20 of these would be a realistically sized light artillery unit.

As Subs is saying, compared to the massive armies we all field in a conflict, 4 guns is just stupid. Heck, let me grab a few friends and some bystanders for cannon fodder. 40 men could take out 4 guns, even when grapeshot is being used.
Heavy artillery, keeping 4 of these:

is very reasonable, but the advantage of a Light Artillery regiment is the speed it can be deployed at, and the large amount of small rounds that can be sent over a battlefield.
Shmuel, keep in mind this is a cosmetic change. I'm happy with the way that subs is adjudicating artillery strikes, and he has already stated it won't change. The only thing that will change is the number of cannon considered to be in an artillery battery/battalions. (I call them battalions, due to the number of men I assume would be in a unit)
This is (approximately) what the light artillery battery would look like:

That's military, my friend.