I don't see the big deal with riding a four wheeler while pregnant. You could "what if" the situation to death but what's the real harm in it if you're driving sensibly? The choice falls into the comfortability of the parent but I see no reason to judge someone negatively for chosing to do so unless they are being totally irresponsibe.
What else can't you do when you're pregnant becuase there is some risk associate with it?
Ride a bicycle? You might fall off.
Drive a car? Someone might wreck you, deploying your airbag and tightening your seatbelt.
Walk down the stairs in your house? You might fall.
Take a shower? That tub could be slippery.
Exercise? Have sex? You might bounce that placenta around.
None of the above are necessary for survival and could be considered recreational activities.