The limiting factor on DWV pipes isn't the size or number of fixtures
it vents, but rather how big around it has to be to prevent frost-up
during the winter.

According to UPC, a 1-1/4" pipe can only vent one "fixture unit", 1-1/2"
pipe can vent 8 fixture units, 2" pipe can vent 24 fixture units, and 3"
pipe can vent 84 fixture units. The size must be increased if the vent pipe
is more than 40 feet long, and a toilet requires a minimum 2" vent.

("Fixture Units" vary from 1 unit for a bathroom vanity, to 4 units for a
toilet).

So, if you have four 1-1/2" pipes penetrating the roof, a safe estimate
would be to assume each is maxed out at 8 fixture units (unless you want to
calculate exactly what fixtures are on each vent pipe). To combine these
into a single larger vent, you would need at least a 3" vent through the
roof (4 vents x 8 units = 32 units).

Realistically, a house with 2 full baths, laundry, and kitchen sink with a
dishwasher would total about 18 units. A single 2" vent would theoretically
be adequate for that case, but I would still opt for the larger vent.
There's no harm in going larger, but going smaller could cause a variety of
problems. As Goedjn mentioned, smaller vents are more likely to ice up in
cold climates (though based on the original issue of multiple small vents,
I doubt that's an issue in this case). A larger vent will allow better air
flow and is less likely to clog with debris.

What you might have a problem with is reducing the pipe-size
as you go down through the system.

Going "down" in the vent sizes is just like going "up" in drain sizes. Just
think in reverse. You should have no problem finding reducing tee's and
elbows to combine the smaller individual vents into a single large vent.

Anthony