Pro players 'try it out'? XD Try something out? Guys with 11000 hours in this game try gun out? Hahaha. You should download workshop map for recoils, you'll see that both M4 have similar spray patterns. M4A1 is good in lurking for example, for taking sneaky positions on sites, long range. On some maps like Nuke, Train A1 is usefull (for 1 player in team, rest should have A4 because of ammo and fire rate)
A4 just have higher fire rate and 5 more ammo that's why most pros use A4 (mostly because of fire rate because on that high level of gaming 20ms is a difference.