What kind of riding do you do? What are your goals?
The short answer to your questions are:
- Yes, there are most definitely 400 watt FTP riders. They’re not common, but they do exist. You can check out this thread for some TR user stats.
- Could you stay at 100kg and be a 4.0 watt/kg rider? It’s possible, whether it’s possible for you is based on a number of factors but probably the largest determinant would be your commitment to training over at least a couple of years. Dedicated, focussed training, nutrition and recovery week-after-week-after-week.
- Does your body like being at 100kg? Right now, it seems like it based on your overall lifestyle, but if you changed your lifestyle I’m sure you could adjust to a different weight. Shedding a lot of muscle is really hard compared to fat. You’d have to really want that. I’m 2 cm taller than you and 30kg lighter (and I look big compared to people I ride with). On even a small incline I would drop you like a bad habit, but if you got up to a 400 watt FTP on the flats you would smoke me (and pretty much everyone else).
Which comes back to, what are your goals? Do you want to be the strongest cyclist you can be? Do you want to be the fastest cyclist you can be at 100kg? They are very different goals. It also depends a lot on your riding conditions. If you live in a flat area, your focus should be on power / aerodynamic drag. If you live in a hilly area then it’s watts / kg.