Have a s4 13bt and when i drive nice and don't give it more then 50% power staying under 1psi it acts just fine and runs smooth (ya my car dosen't boost unless i give it more then 50% above about 3500rpm) and if i push the peddle down a little harder around 3500 or up and i go from vac to boost just about where the gauge reads 0psi my car flat out losses power and make a gargle sound and rocks a hard. tell i let of the gas so i have been driving nice and shifting at about 4500 at the most. it's not the 3-4k hesitation from ground, we checked them and there all good

any idles what could be making it do that?

running a greddy front mount with hks ssqv, stock turbo, walbor 225 rewired to full 12v, RB fcd, RB 3" catback

 

If you're running lean like I suspect, be very careful until it's fixed.

Check your FPR vac hose.
If the FPR vac hose pops off, it will keep fuel pressure from rising with boost.