If someone has 4 years of high school Chinese, what are their chances of getting assigned to that language if they enlist for 35P?

If you are looking to be a Chinese Mandarin 35P It's not likely at this time. The MOS is pretty much at the numbers they require, and slightly over in some skill levels, so they aren't placing Chinese at a priority.

I got my language before basic, but this was years ago. See if your recruiter can put you in contact with the reserve component linguist manager for your state. That's who got me Arabic. Mind you I only had 2 real choices at the time-Arabic or Korean, and Arabic is more comparable to the Spanish I already spoke, so I picked it. The needs of the Army come first so don't get your hopes up for a language we aren't missioned for. Other branches have other focuses, so that mau be a route to consider.

You can do it but I don’t know the route. I just know that I saw it in AIT and I know reserve soldiers that do it. They just take the dlpt and score high to scip DLI altogether. If you think your proficiency is good enough, then do this.