I would probably use Scott's suggestion and make this a 2nd level variant of the web spell, especially with the rather limited area of effect and duration.
I set the level of the spell at 4th because of one main point- the spell's use is NOT limited to the caster alone. An entire gang of thieves for instance can make use of it, which to me makes it a very useful, and therefore powerful spell. I vasilated over making it a 3rd and 4th level spell. Either would suit, though I decided to er on the side of the higher level.
