What was your FTP when you started and where did you end up? Also, when did you start and end your winer training (time of year)?of course it all depends on your level, experience, objectives, time available, etc.
I would not take the 20% as it is, i-e I would not hammer the crap out of myself doing 20% high intensity during a week.
Riding at L3-L4 intensities will get you good at that: be fast arround threshold, thats it. You got to throw in some high intensity stuff. But, to each his own.
FYI, I basically did no L3-L4 this winter, or very very little of it. All I did was L5 work, high intensity micro-intervals, a few short spins, all on a 8-12h training regime, on the trainer. My FTP got to an all time high. Of course, you might want to still do some L3-L4 work here and there, I just feel it is not worth the efforts, fatigue, recovery, mental stress, for the gains they will bring.
Like everything else, you will probably see some gains doing L3-L4 work, but they will likely plateau relatively quickly. Variety is important.