Poison Touch's cast range increases by 100 per level, starting at 600, and ending up on 900 on level 4.

However, the range of the effect doesn't increase along with it. It always stays at 500 range. This means the cast range increase kind of pointless. On level 1, the spell can target enemies about 500 range behind the primary target max range, but on level 1, it only reaches 100 range behind the target when cast from max range.

Just recently, the effect range was fixed to increase with cast range bonuses. Aether Lens and the +125 range talent do make it reach further now, but the spell's own cast range increase per level does not increase its effect range, which is really weird and doesn't seem intended.

Imagine if Magnus' Shockwave would have increasing cast range per level, but have the shockwave always travel the same distance on every level. This is currently how Poison Touch basically behaves.