Hi,
First, I want to say I really wish you can manage this.
Having said that, I think you should acknowledge that you have a fairly big gap in front of you.
I have been aiming at 4'00'' the past 3 years, and once you get below 4'05'', the progress can be incredibly slow. But also maybe you are still at a phase where you are progressing faster than me. Currently I stand at 4'03''80.
I'm also saying that because your PBs in other distance do not seem to indicate current ability for 4.
I do think I'm fairly homogeneous past 1500m distance, but my PBs are 4'03'', 8'58'', 15'25'', 32'57'' (with 3000m that could be improved), so slightly better than you, and I still fail for 4'.
For a 1500 runner, I've always though I lack speed, but I still can do 55'' (unofficial relay leg).
I would work on your base mileage as you are doing currently, with a bit of pure speed once a week, but I would cap at 100km/week. Then start some race specific training something like 2 to 3 months before.
Good luck!