It won't prevent you from becoming partnered, but it will do nothing for your channel. There is no point doing sub4sub because you are left with dead subs.
For an example; You do sub4sub, gain lets say 150 subs from it. You post a video.. and only 2 people watch it. You think to yourself "I have 150 subscribers, why does my video only get 2 views?"... The answer is because those people don't care about your channel, they only care about seeing their subscriber number go up. If it makes you feel better knowing that majority of your subscribers don't watch your videos, then somethings wrong.
I'd rather have subscribers that actually watch my videos, not just subscribe so they can get their subscriber number +1.
Doing sub4sub will not help your channel in no way, shape, or form.