Lars - Going from analogue to digital is a big change. The racing is different and so is the track design - especially the placement of lane changers and pit lanes. With such a big layout possible, I would suggest getting advice on a busy digital forum, like on SlotForum. There are more digital enthusiasts there with more experience of designing and building digital layouts. They will all have different opinions!
Having said that... we have now designed and built twenty-six different Scalextric digital layouts at WHO/digital, all around the 30 metre size. We race six cars on (mostly) two lanes. Each time we learn what features work and what features could be improved. Some things work because they are thought-out and planned, some of the best things are a surprise!
Four digital design elements that might be useful to you are:
1) Aim for a straight lane changer (known as an XLC) at the end of a straight, when the cars are slowing down.
2) Look at the 'racing line' on a track and place the pit entry away from that prefered lane - full-speed racers don't want to encounter cars slowing for the pit lane. Add an additional third lane (or create an early pit entry) to remove pitting cars from the racing action. Likewise with the pit exit - avoid cars entering the racing line, especially on a fast straight. This is where a three (or four) lane section can isolate racing from pitting. Ensure the pit exit pieces are used to remove the non-racing lanes - cars will need to slow a little as they change lane.
3) Again, looking at the racing line, choose places where an XLC would allow cars to change lanes to stay on the line. For example, a short straight between a left-hand turn and a right-hand turn is ideal for an XLC. There are also old curved lane changers (know as CLCs) that are brilliant for keeping that racing line. The in-to-out changers work best for us. However, the CLCs need modifying to use with ARC Pro and are only available as R2 90-degree curves.
4) Leave some space between lane changers - I'd say at least one and preferably two metres on a big track. You need to hold down the lane change (LC) button before the XLC, so - if too close - you might catch the earlier (unwanted) XLC or pit entry if you press the LC too soon. That is annoying! Likewise, with ARC Pro - leave space between the powerbase and any XLC or pit entry piece. Holding down the LC button over the powerbase causes problems with sensing the car.
Specifically on your lane question - I'll answer as best I can...
Having four lanes and the '2 metre rule' means that swapping from lane 1 to lane 4 requires three lane changes and 5.5 metres track length! If you use the '1 metre rule', then it is down to around 3.5 metres. If you used the XLC next to each other, it is 1.5 metres, but you are likely forced into either three lane changes or no lane change - it would be hard to control and guarantee a change from lane 1 to 2 or from 1 to 2 to 3 (but not 4) if the XLCs are so close. For me, that makes the 'racing line' experience less satisfying than with two lanes.
Ultimately, with 4 lanes, you are forced to compromise. For example, you might designate a 'slow lane' round to the pits and effectively race on three lanes. You might alternate XLC connections at the end of straights - on the first corning using one XLC between lanes 1 & 2 plus an XLC between lanes 3 & 4; then at the next corner an XLC between lanes 2 & 3. So a racing line strategy would need to be planned and executed over three consecutive corners to get from the outside to the inside. Of course, any overtaking and full-on racing action within that three corner section would wreck the plan.
Finally, I think I've touched on the "how often" question. You must be able to plan and execute a lane change (without any 'unwanted' changes). When strategically placed, 'less can be more'. It is a case of experimenting - designing a track suited to digital racing and then racing on it - constantly refining the placement of XLCs and pit lane pieces until you are 100% happy that the track suits experienced racers and also people who are digital racing novices.