Heathrow to Gatwick

Does the 4 hours include the time you need at the airport for checking in to the following flight? Four hours allows you very little leeway. Are you flying internationally into LHR (and out of LGW) - all of which make a difference.

The National express coaches run approximately every 10-15 minutes between the two airports - journey time of 75 minutes, costs around £25-30.
You might be able to gain up to 30 minutes by having a pre booked taxi - but it will probably cost >£100.

If your plane lands on time, and you get through immigration relatively quickly, terminal 3 is close to the bus station, so it might be possible to Gatwick by 2:30 pm, leaving you 90 minutes between flights Any delays are almost certainly going to scupper those plans.

I’d imagine 4 hours gives you plenty of time. If you were to take the Heathrow Express to Paddington, then hop the Circle Line to Victoria Station, then take the Gatwick Express from there, you’d be looking at maybe an hour to hour and a half of total travel time. So as long as you actually landed at Heathrow close enough to on-time and made it out of the airport and on to the Heathrow Express in a timely fashion, you could easily do it and not even have to do any frantic running or anything.

There’s other options out there also (taxis would be much pricier, some bus service options exist) and truthfully even with some traffic you could likely do it with time to spare. But I’d imagine train is the best and safest option.

Edit - Also, is your itinerary a single booking? In other words, did you book a single ticket and the airline gave you that 4 hour layover between airports? Or did you book two entirely separate tickets yourself? That makes a big difference - if it's the airline that did it, that gives you some added security.

Is it a European or domestic flight or a long haul? It'll take about 1.5 hours by a combination of walking, tube and train whilst Google provides an optimistic 55 minutes by car via the M25. I have never had a journey on the M25 (there have been many) where there wasn't some form of congestion so I would hedge my bets and suggest 1.5 hours by car just to be on the safe side.

If it's a European flight you need to be at the airport at least 2 hours before departure, 3 hours for long haul flights.

Are you counting your four hours from being at Heathrow entrance ready to go or does the clock start from your intended arrival time? The difference between the two affects your 4 hour window dramatically.

It may take you 2 hours to get out of Heathrow depending on the length of the queue. Allow at least 1.5 hours by a booked car as there are always holdups on the M25 plus time to get to wherever the car is parked in the multi storey car park (add 15 minutes). You will need to be at Gatwick at least 75 minutes before your departure if it’s to Europe and 3 hours if it’s to America.

It’s too tight for comfort unless you are booked on a through ticket. When is the later flight from Gatwick? Same day?

I would not try to travel between the two by tube and train, but instead use one of the direct bus services.

I would gladly pay extra for a trip that does not involve travelling between airports on the ground. Especially for a connection that does not involve travelling between airports in London. Traffic on the M25 can be running smoothly, miracles sometimes happen, but it might also be almost stopped for parts of the trip.

But if you still want to do it, I'd check with the airline what happens if you miss your flight due to delays on the ground. Will they put you on the next one or do you have to buy a new ticket?