I started working as an independent contractor, but sold more then the gym realized I could sell. The first two weeks, as agreed upon, I was paid the full check... then the last two weeks, they are stalling on the check, and wanting to have me fill out a w-4 and pay me half what is owed now... please help! Thanks

Did you fill out a W9 when you started as an independent contractor? If so, they can't change your status. They can't pay half of what they owe. Ask them why they are doing this. It doesn't make sense as they save money on taxes if you are an independent contractor.

If you are in an at will state, they can terminate you at any time. So if they want to change the job strucutre, they certainly CAN do it.

They would still have to pay you as a contractor, up to the date of the switch.