4th order (active) filter, I want constant phase shift band
There are two ways to approach getting a filter with a flat linear phase passband.

a) Flatten the phase of a conventional steep filter. You can design all-pass phase correctors. You don't say what your 4th order filter is, some types have flatter phase than others. It's easier to flatten the first half of the passband where the non-linearity is small, it gets harder as you near the edge of the passband.

You need to start out with as flat a passband phase, which means as wide a passband, as possible, while still meeting your stopband requirement. If you're not using stopband zeroes yet in an elliptic design, then you have a lot of scope for widening the passband, while keeping your attenuation to specification.

b) Flatten the amplitude of a linear phase filter. These have a fairly soggy stopband, which will almost certainly need to be augmented with stopband zeroes. There are published designs for this for up to 8th order, so the basic design should be no trouble. 

Do you have a stopband specification? If not, that's the first thing to fix, because otherwise, how will you know how wide you can make your passband?