Well you can if you've ridden it. If not, we're still on the A49 and I can't go cos I put us there.
And (as far as I can see) the only other A/B road that the B4399 connects to is the B4224. Luckily I have ridden it (the B4224) at the Mitcheldean end. That gives us as options the A40, A449 and the A4136 to Monmouth.