Why do you want to use the 4% and not the 1.9%? Or do you not have 1.9% either
4% is for vibrant reds, everything else you can use with 1.9%
I can't see there being a problem mixing it with that developer. But using a colour touch plus tube with normal colour touch developer may stop it from covering all the white hair. So i would get some more 1.9% and save your CT+ developer, unless you really need it now
hope this helps x