Let's see:
Wavelength at Eb2 at 20C and A4=440Hz....441.4 cm
Wavelength at Bb1 "" "" = 589.2 cm
Length of 4th valve branch = 147.8 cm. or about 58 inches. To get each leg length, subtract the length of the path through the crook and halve the result. E.g., if your crook is 5 inches long, 58-5 = 53 inches /2 = 26.5 inches per leg. This will probably be a bit long, but it's a good place to start.
If you want to check your numbers, just get a hunk of plastic tubing of the right diameter, substitute it for your 4th branch and tweak the length until it sounds right.
You can find a very nice little musical calculator for free download at
http://www.bias.at
