Street Magic pg 118.
Whether positive or negative, in game terms background
count reduces a character’s Magic attribute by its absolute value.
...
If background count reduces
a character’s Magic to attribute to
0 or less, he is rendered unable to
use any magical abilities within the
area. A background count-modified
Magic attribute counts for all uses
of magic, including dice pools and
limitations imposed on the Force
of spells or spirits. Additionally, the
process of gathering and shaping
mana is more difficult in areas with
background count, so the absolute
value of the background count is
also added to the Force whenever
a character resists magical Drain.
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So the ansewer is that without overpowering you are limited to force 4 spells, but you cast them as if on force 5 for drain value purposes.
There is an example on pg 120