They aren't - they will still be working long hours as was made clear in the link
The title of this story is misleading. The city traders are not saying "we want to work 9 to 4", they are saying "we want the markets to be open between 9 and 4 so that they can then work slightly less insane hours.
The title is in quotes, so I'm assuming you copied it from the BBC article headline and then they subsequently changed it to "City traders: We want shorter working hours" to make it not a lie.