This is the conclusion I reached after analyzing Strava data from riders in the 4 and 5 range
3 to 4 days a week
2 hours on the trainer at power above the 3 range
2 days a week
2 hours on the trainer at power below the 3 range
Do this and you should reach 4 sometime in May
I wrote this while reflecting on my own training
Please take it just for fun~