Most of the time, your shutter speed should be set to the flash sync speed for your camera (i.e. about 1/250). A strobe's actual flash duration is shorter than this (and if you do anything high speed, this is a number you need to look at), but using the sync speed will allow the flash to go off while the shutter is open. Using a faster shutter speed can result in no exposure or a partial exposure. Longer shutter speeds will get the flash plus ambient light.
Unless you have a light system that will talk with the camera (i.e. a Nikon and a bunch of their speedlights), your camera's meter won't do jack for proper exposure. You can get it a few ways, one is calculating it manually based off of guide number, light distance and all that jazz. I don't know all of the formula, so I am not even going to try and get it for you. The second is to use a dedicated light meter. Really handy, does a great job, and all around an excellent tool to have. Wouldn't be surprised if you can find one that does flash readings for under $100 at KEH or eBay. The third, since you are shooting digital, will be to chimp and just take photos until you get there. Much longer than just using a meter, but still possible.
How is your light being triggered from the camera? If you did a 4s exposure, and it came out like that, I am pretty sure the strobe just never went off.
Also, a note on adjusting exposure: you will leave shutter speed and ISO (most likely) static, instead adjust the exposure by aperture or flash intensity. Flash intensity can be controlled by lowering the power of the flash, or moving it further away.