If you happen to live in London, Birmingham or Manchester, we have good news for you. Three will be offering you 4G for free - as long as you are a Three customer, obviously.That's the promise as the network attempts to overshadow the launch of O2 and Vodafone 4G services this week by highlighting its own proposition. According to Three, all customers who have a 4G-ready device will get a 'simple software upgrade', allowing them to take advantage of the 4G network. No change of device, no contract to sign, no new SIM - it's that simple. Three will also maintain its All You Can Eat data policy too.Of course, you'll need to be in one of those cities to get the benefit, although Three is promising an accelerated rollout after that, with the aim of covering 98 per cent of the population by the end of 2015.If you want to sign up for a deal, you can do that now. Prices vary according to the handset obviously, but don't have to cost a fortune. For example, if you pick up the Huawei Ascend P2 pictured above, that's available from £20 per month. As a 4G phone, it will be shifted onto that network in December, making it quite a bargain in 4G terms.