Powering 4 12V 0.45Amp fans off USB?
The piece of knowledge missing is the relationship between voltage, current and power, \$ P = VI \$.

From this we can work out:


One fan will require \$ P = VI = 12 \times 0.45 = 5.4 \; W \$.
Four fans will require \$ 4 \times 5.4 = 21.6 \;W \$.
Your USB port can supply \$ P = VI = 5 \times 0.5 = 2.5 \; W \$.


You haven't enough power to run one fan. Introduce a boost converter with an efficiency of, say, 85% and the situation becomes that much worse.