From personal experience...
If your guitarist is playing in drop D or C and you're going to be matching the riffs, a 5 string won't have you covered. Sure it will give you access to the notes, but it will likely be stiff and unconvincing, like a session player sitting in - playing the right notes but not inhabiting the riff.
If you're playing around a riff in drop C, you need to have that big open note to bounce off - constantly playing off the first fret is really going to hold you back. When playing in this genre you often need that open root drone, and also need to the freedom to improvise around underneath the guitar solos. Not that every riff or solo will be in the same key as the tuning, but it generally is 😁
I also found that the looseness and rattle of a drop-tuned 4 string is helpful as that is exactly how the music should sound - stiffness and pinpoint accuracy is not what it's all about. There should be grease and swagger.