The hit poison area is not effected by your poisoning skill on the serpents fang. It's just like hit fire, energy or physical area an any weapon. The area effect is spread around a few tiles from you as you use the weapon. For instance, in PvM everything around you will take poison damage as you use the weapon. For this reason, you will see the weapon used a fair bit in PvM but rarely in PvP.
Your poison skill, is used for the special move infectious strike. Since the serpents fang is a kryss, if you apply poison to the blade, you can use that special move and get a chance to increase the level of poison that is on the blade.
Yes, poison level of poison fields are determined by your poisoning skill, but also magery. For lvl 4 fields (poison skill + magery) / 2 must be 100+. Since you didn't mention your magery skill level, I can't tell you what lvl your fields will be.
Hope that helps,
Q