We're now ending our live coverage of this event - but before we go, here's a recap of what's happened today.
- Google and Facebook have faced claims of bias and being too intrusive over the way they study their users' online habits.
- Apple was accused of unfairly treating developers, who can only offer iPhone apps via its own App Store.
- And many of the questions targeted at Amazon centred on why it isn't doing more to combat counterfeits.
And on Friday, our Tech Tent podcast will further explore how the four chief executives fared.