The HD 4000 is faster than 520 card, if you have an ivy bridge already don't upgrade to the 520. Generally The 4000 couldn't beat the 6550D in gaming performance and compute, so in other words 6550D is faster.
More details, see these pictures here:
But of course Ivy Bridge has more processing power
, don't be fooled! If you are looking more from Integrated Graphics performance, you can wait for the next gen APU from AMD rumored to be released on August.