Collet extension

[Cosmo52]

Hello all, any experience here with using a collet extension on a 1/2" router?  I am considering to use a router and template to make a corbel on 8 by 12 fir timber.  A youtube video shows a fellow roughing out corbels with a jigsaw and finishing with a router and straight cut bit.  I am having trouble finding a router bit that will reach 4 inches (half the distance of the beam), so a collet extension might be a solution.