The current two toilets, which have an 'S' trap has a vent pipe (2 inches) running
Directly from the S bend, out the exterior wall and up to a vent pipe to the roof. Also there is a 4 inches vent stack pipe about 20 feet away from the toilets. Is this vent pipe still required if it's replaced by new toilets?

Any advice would be greatly appreciated.

All toilets have an intregal trap built into the toilet. No need for a trap under the toilet(actually not allowed.) However, you still need to have the toilet waste pipe vented. Maybe if you could show us a picture of your current settup, or draw up a diagram we could help point you in the correct direction.

Where do you live, this is not to code in any of the USA