the 4 would be the base, they could try mixing
1/4 3
1/4 4
1/2 552
that would give a better coverage and more depth with out going a whole shade down, its a bit trial and error sometimes....although u could try a whole shade down with
1/2 3
1/2 552
but u could risk goin a little to dark...but thats probably the lesser of two evils in ur case.
or u may be able to stick to the 4 and go with a 452 if such a shade exists within their colouring system.
the problem with white hair is that it grabs the fashion tones i.e the reds and plums, the general thing is that when theres alot of white you need to go down a shade from ur ideal to get it to the colour u would like, the ratios need to be at least 1/2 base shade to 1/2 fashion tone.
why dont u tell them ur not happy with the outcome at the moment, and tell them what we'v suggested, if uv been goin for 7 years ur obviously happy there bar the last couple of times, it seems a shame to loose their services, why not give them one more chance? hth's