Yes you can, with caveats.
If you're just doing half rounds with no beads or coves, it would be fairly easy to set up with a moulding toolpath - use a half-round as the profile vector. Would need to use the larger area clearance tool option to avoid breaking your ballnose bit.
HOWEVER - at 4" thickness there is a very good chance that the collet will hit the side of the part when the carving bit is at the bottom of the part unless you're using fairly long bits. It would make sense to measure and draw the collet and bit in advance to see what your side clearances will be next to the half-round. Also remember long bits are more prone to vibration and chatter.
If you do have beads or coves you'll have to get creative with clip art.
Or, you can model it outside of VCarve Pro, save as an STL and import, assuming you have software that will do that. Others can talk to free and paid options.
D&S Artistic Woodworking http://www.dsartisticwood.com