Hi moonnightingale!
First, (j)^2 = -1 => 1/j = -j, also the unit of impedance is Ohm.
Second one: (-j4000) is in parallel with 4kOhm, it likes resistor (-j4000 ohm) is in parallel to another 4kOhm <Because the concept impedance and resistor is quite same in this case>.
The third one: he can write 0 degrees it is clear that Vs = 0.750 cos(5000t + 0 degree) where 0 degree is the phase, and the point you highlighted in the last formula presents the phase of Vs (It is only the concept).
Hope this help!
Duc.