I've posted on this matter a few times here. It seems the very high numbered, ph2 Williams is an Italy only thing (not France). The numbers are usually high 5k / low-high 6k. I do not recall ever having seen a Williams numbered in the 4k, which almost makes me think the original run was ~3.8k (if you look in the register, the last ph1 numbered car here is 3806), with Renault Italy then doing another run, most likely starting with a 5xxx. So everything seems to more or less stack up with the ph1 numbers.
I've personally added one numbered ph2 to the register, number 6878, very nice and cared for car
Also seen photos of a supposedly new, never registered ph2 - no. 6206.