Is this to create a rounded groove/flute (The opposite of a bead)? Much depends on location, but the groove could be cut using a scratch stock or round tipped plough/plow plane cutter. Terminate each end of the cut using a gouge of the correct size or a shaped piece of timber wrapped in abrasive paper.
Another method would be to round an edge on a 4″ x 1/4″ wooden slip and edge it with sandpaper. The groove can be sanded – guided by a clamped fence- and ends terminated using this method.
Much depends on the number of grooves you need to carve/cut.