One could google the imperial to metric conversion instead of asking for someone else to do it on a Forum 
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To the point of this thread:
I gotta disagree with the call for large amounts of volume.
4Wkg can definitely be achieved on something like 5hrs of riding a week.
Ultimately, the FTP is determined by a short test, not over a 4 hour ride.
If your genetic ceiling is at or below the magic number, this might be a different story. I however believe, that there are far more young, healthy, male cyclists out there who could realistically achieve an FTP beyond 4Wkg, than people think.
I will say that most people over focus on FTP. I have hardly improved in FTP (based on test results) since Nov 2020. I have however become a much, much better cyclist, being able to get much closer to my best power when racing in the mountains, or racing iTT.
On the other hand, I do understand why improving FTP is desirable.
It’s easy to measure, a good (not great) way of measuring progress and fitness, and… let’s be real, easy to communicate.
Cycling people I meet always ask „what’s your FTP, bro?“ and with just 3 digits (as an absolute number) I can convince them of being super fit.
For all of the non-racers/ non-pros among us, I still find FTP useful.
I am a numbers guy, and I am also obsessed with marginal improvements. Riding fast, feeling fit, and yes, being faster than others, has kept cycling interesting for me for years now, and is an important and motivating part of the sport for me, that gets me off the couch and on to the turbo, even after a hard day‘s work.
Sorry for rambling on for so long.
What is needed for a 4Wkg FTP?
My 2 cents:
I see it as an equation with 3 variables.
- Fitness
- Weight
- ability to access your fitness when needed
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(Edit: I didn’t type the (4.) nor can I delete it.
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As a relative novice to the sport, weight training might not be a bad idea to incorporate into training. When racing, it is very common to do that during the winter months, to focus more on specific on the bike training during the season.
When not racing, it doesn’t really matter.
Weight training will definitely improve fitness, and can not be substituted with stuff like big gear work.
The most time efficient way of building on the bike fitness will be intervals. I’ll probably not make a lot of friends here, but polarized training is very time efficient, and helps to Prime for the pain of a ramp test.
What ever Route you go here, I don’t think that riding 15 hours a week is at all necessary to reach 4Wkg.
That is of course, if your hours riding are spend training. Riding a bike and training are not the same thing. Training is riding to zones (power in this case). This is what gets most people to 4Wkg and beyond. Riding around for hours and hours can help you improve, but it is extremely inefficient.
Rest is also part of this equation. Monitor how much you sleep and how you feel. Sleep is not only important to build fitness, but also to stay healthy and mentally sharp long term.
That’s another reason why I don’t think overdoing the „base miles“ is a great approach. It can build up a lot of physical stress, takes up a lot of time that could
otherwise be used for rest, and builds relatively (time to outcome) little fitness towards your FTP goal.
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weight… ride your bike, eat high carbs, high on veggies, fuel your work outs (so eat well before, during and after your workouts), keep fats and protein relatively low. You’ll reach a good cycling weight very likely just by doing that.
I myself tried starving myself early on to embrace the cycling look, and ended up becoming significantly weaker as a cyclist. For me, 71 to 72kg (I am 6‘3// 191cm) has come out as the sweetspot, and 68kg (where I once was) has hurt me mentally and physically.
Don’t over Stress the „being light“ stuff.
A high absolute FTP is significantly more important than a super low weight. You need food to perform, the rest will usually fall into place.
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being able to really kill yourself during a test is an important skill, that takes practice. It will also be a relevant skill when you get into racing.
Many people don’t reach their physical limit in tests or races, because they crumble mentally. After the test/ races, you usually feel like kicking yourself for not having dug deeper.
For me, intervals have really helped me. This is also where I think sweetspot work has its‘ place. A long sweetspot or threshold interval is extremely mentally tough to endure. With that mental challenge on a frequent basis, an FTP test will be much less of a shock to the system.
Also, find something that motivates you. It can be loud music (which has been proven to enhance performance), imagining yourself winning a mountain stage, idk. Through riding more, you’ll likely find something to give you that extra edge.
Lastly, break efforts up into segments that seem manageable (not literally break your efforts up, this is a mental exercise of how you view what you have done already and how much you have left).
This has helped me a lot. Not thinking „oh no, it’s 95% left, I have done nothing yet“ but more like: „only a little longer and I’m at 10% - let’s see from there.“
Hope this helps a little
Its just as valuable sometimes to focus on the ‘process’ of getting faster and see where that takes you!