If the hair is a warm level 4 then it might take a couple of attempts to get the undercoat light enough for sliver.
The best product I have found for creating silver is Luocolor, I find I can get a true silver tone using 3/4 P01 with 1/4 ash shade. A few days ago I added 7.1 as the ash shade and had a really perfect silver/ grey that was still reflective and not dull (about a level 9) I have also used 5.1 in the same mix which was a more cold steel tone (about a level 8). This is a permanent colour so great for full head lightening but you would need to watch it if there is virgin hair there.
Using black in very small amounts does work too but can have a slight blue undertone. I have used this with DiaRichesse before when the hair was only lightened to a level 9 and the blue undertone took any any warmth to a very cool grey. You have to check with the chosen brand, L'Oreal tend to have a blue undertone, but some brands will have a more green undertone.