I want to start a garden but the soil is filled with crabgrass and weeds. I dug the top layers off but as the lower layers got wet new grass/weeds sprung up. Can I poison the soil to get rid of it all? Do I just dig deeper? Will the grass and weeds die when I put 4inches of good soil on top?
This is in southern California so it is mild climate.

You can spray the area with a grass and weed killer. Make sure you follow the recommendations about the time you are to wait until you can plant after spraying. I can't remember the name of the one I used, but it suggested waiting 2 weeks before planting.

Round-up! That's what I use with great results, providing you don't get it on plants you plan on keeping.
I'm in SoCal too, so our weed season lasts all year long. It's been my best friend in my uncultivated areas.