You're on the right track, but there's more to it than just 8x4. Here's the breakdown according to the NEC (National Electrical Code)

NEC 314.28(A)(1) Straight Pulls
General Rule

For straight pulls, the minimum length of a pull box or junction box is 8 times the trade diameter of the largest raceway entering the box.

Multiple Raceways

This applies to the largest raceway entering the box. The other raceways don't add to the calculation.

Therefore, the answer is 8 x 4 = 32 inches.Important Considerations



Minimum Size

Your pull box MUST be at least 32 inches long.

Raceway Entrance

Ensure that you comply with NEC 314.28(A)(3) regarding raceway entry. All raceways must enter the box on the same wall.

Manufacturer Instructions

Always check the manufacturer's instructions for the pull box you're using. They might have specific requirements or limitations.

Local Codes

Your local electrical code might have stricter requirements than the NEC. Always consult your local authority having jurisdiction (AHJ).

Practicality

While 32 inches is the minimum* required, consider the ease of pulling the conductors. A larger box might be beneficial for easier handling, especially with multiple large conductors.

In summary

Your minimum pull box length for a straight pull with (5) 4" conduits is

32 inches. But remember to check for local amendments, consider practicality for easier pulling, and adhere to the manufacturer's instructions. Always consult with a qualified electrician if you're unsure. Flag for review