4" pvc, 18" long so 1:4.5 ratio
The ideal ratio is 1:1, the pipe length should equal the pipe diameter. (I am assuming that the question you asked is the one you want answered.)
But, that ratio is really pretty unimportant. People rarely build guns with the ideal "chamber length to chamber diameter ratio", mostly because of the difficulty getting large diameter pipe. Bottom line, for the chamber, use the largest ID pipe that you can get (pressure rated) taking into account you also need the various fittings. The fittings should also be pressure rated but for a 1x gun they often aren't.
The usual ratio people design for is the chamber volume to the barrel volume.
Once you've choosen the pipe diamter for the chamber choose the length of the chamber based on the barrel diameter, barrel length, desired CB ratio, overall gun length, gun layout (inline vs. over/under vs...).
A 1:1 chamber volume to barrel volume is usually OK. Figure out how long you want your barrel, then based on the barrel diameter calculate the barrel volume then use that to calculate the chamber length.
A quick calculation would go like this;
The correct chamber length, L<sub>chamber</sub>, for a CB of 1:1 given the chamber diameter D<sub>chamber</sub>, barrel diamter D<sub>barrel</sub>, and barrel length L<sub>barrel</sub>;
L<sub>chamber</sub> = L<sub>barrel</sub>*(D<sub>barrel</sub><sup>2</sup> / D<sub>chamber</sub><sup>2</sup>)
If you want to calulate the chamber length using a CB of something other than 1, the equation is;
L<sub>chamber</sub> = CB*L<sub>barrel</sub>*(D<sub>barrel</sub><sup>2</sup> / D<sub>chamber</sub><sup>2</sup>)