Your lighting kit is pretty weak in terms of the amount of light it gives off, so if it's your only source of light, that's why you're underexposed. You're essentially using 4x45W lightbulbs for continuous light that they say is the equivalent of a 200W incandescent bulb. While this is generally higher than most ambient lighting situations it's not higher by much. This would, in my opinion, still qualify as a low light situation, so at 1/125s and f/8, you probably need to be cranked up to an iso over 3200 to get a decent exposure. And you certainly won't be doing a lot with lighting ratios at all. This lower level of light can be good for video, since you're trading off for continuous light.
But flash is generally favored for stills portraits because in that short flash burst, you can get a lot more light--enough to create lighting ratios between the subject and the background that make for more drama. With a speedlight flash at nearer distances, you could easily use iso 200 or 400 with f/8 and your camera's sync speed and get good exposure.
The camera needs a lot more light to see by than your eyes do.