The group will release the follow-up to 2011's 'Build A Rocket Boys!' next March, the same month the frontman reaches the milestone age, and the singer admits he has been exploring the ways he and his friends reacted to the landmark birthday.
He said: ''The record might be called 'All At Once' because you are all ages at once at 40. Society tells you that you have to live in the moment, so you see guys at 40 looking more thoughtful than anyone older or younger.
''A lot of my friends are discovering ecstasy as they near 40 and I'm going, 'What the hell are you doing?' We were on the fringes of the E generation but we didn't get into it.
''But how does a rock star have a mid-life crisis? Sell his motorbike?''
The 'Grounds for Divorce' hitmaker is pleased with the record and is finding it tough selecting a lead single.
He said: ''There are five or six songs we're choosing from.''
Although Guy has been hard at work on the new album, he took a break from the band to record the string arrangements for Arctic Monkeys' headlining set at the Glastonbury festival in June and was delighted to be involved.
He told the Daily Star newspaper: ''That was an honour.''
This Doctor won't be saying 'goodbye' in the same way as those who have left the series in recent years.