Good work man! Very nice to improve so quickly. If I may give some other advice to you and other learners: nail and understand the chess basics. Practice tactics- this will develop your chess vision. Study endgames - both among advanced players, but particularly among intermediate ones, a lot of games can be won or saved in endgames
400 to 1000 in 3.5 months

I do not think it is. Now, I only got from 1000 to 1300, so I think I cannot compare formyself, because I was 700 was my lowest rating. But still I think 1000-1300 is pretty hard