My iPhone4 can't get into DFU mode anyone can help?

Not sure why you want to get it into a DFU mode unless it is to restore and you have a passcode set. Here's the way I do it:

1. Start with your phone unplugged from your computer.
2. Hold the Sleep/Wake switch until you get the red slider and turn the phone off.
3. Start iTunes on your computer.
4. Hold the Home button down and keep holding it down while you plug the phone into your computer. Continue to hold the button down until you get a "iPhone detected...". Follow the screen instructions to restore.

the same as upstairs
another way to DFU:
- start with yr iPhone plugged with PC
- shut down iTunes
- press sleep and home button at the same time until apple logo appear
- release sleep button and keep home pressed
- turn on iTunes
- u will see iTunes indicate that u have an iPhone to restore