You need to crop them before you export them and take them to a print shop.
4x6 is a standard 3/2 ratio, 2 x 3 = 6 and 2 x 2 = 4. 90% of cameras on the market use that ratio as a sensor size. There are also 4/3, 16/9 and 1/1 ratio cameras. Which camera do you use, make and model?
More than likely the camera you are using is either a 4/3 or you have it set to take image in either the 4/3 or 16/9 ratio. If you ask the photo lab to print them as they are, No Cropping, you will get the whole image with a larger white boarder either on the top/bottom or sides.
You can crop them to a specific ratio in LR so they fit better on 4x6 paper and contain the image parts you most like.
It really has nothing to do with exporting and or exporting setting. It does have to do with the size, ratio, of the image you are exporting and taking to the print place.
I'm experiencing the same problem as the person who wrote in the query about exporting pics from Lightroom, taking them to the photo lab to print (4x6's) and resulting size is never correct. A lot of the pic are cropped or is too small. I can see the whole picture but only a bit shows up.
I have a Canon 5DMkIII, which is a full frame sensor camera, so therefore I really am not understanding the 4/3 or 16/9 ratio that you mentioned.
I photo lab tech (at Costco) who uses Lightroom mentioned that it's the aspect ratio that I have to set in the Develop module of Lightroom. Then someone else in the lab mentioned it's the use of the Sync button. I've tried both, nothing works.
I'm ready to drop Lightroom and use a different photo developing application. I desperately need help and quickly please.