So, I am in 12/8 and I want some instruments in 4/4 using the same bar lengths as 12/8.

When doing a local input of time signature (alt+enter) I get different lengths (dorico is keeping the quarters at equal value).

How can I get the bars to match?

Have you searched this forum for polymeter? Several helpful threads on this exact question.

There’s also a Scoring Notes article about it: https://www.scoringnotes.com/tutorials/masking-meters-creating-polymeters-metric-modulations-dorico/

It’s not easy, but it works.

Thanks, but such a workaround is just too much. I’ll write it in quadruplets and move on.