There is a lot more to designing a speaker cabinet than nailing a box together and screwing a couple of drivers inside. The box must be tuned to the speaker, even if it's just a closed box. Without this, the results will be hit or miss, the chances of optimum results are very low.
The Sunn folded horn enclosure was made back in the days of "Cut and Try it". Ever wonder why Sunn stopped selling that design around 1970 when they stopped using JBL drivers? It's tuned too high for most modern drivers, the results are boomy lower mids and farty weak lows. Don't even try it with a 5 string. You'll kill the drivers.
First select a driver you want to use that is available and affordable. Download the spec sheet and see what the manufacturer recommends for enclosure internal volume and tuning. If you can't find the info, bug the factory until they give it to you or find another driver.
Two tens and a fifteen is a good combination, but they need to be in separate boxes each tuned for what's inside. It's not easy to match impedances and balance the volume from the two cabinets. Are you sure you have the time and money for all this?
What's an Ampeg B4? I know there is a V4 and a B42X.