Fastest way is not classing your BO and power leveling it but to late for that so you will have to grind it out. It's a tough grind so hang in there. But unless you have a restore you will not reach 900 sd with with at level crew

If you have some BB/CV crew (or credits to get one) you might level faster with premium BB B65 or an event ship (Katsuragi CV or one of the BBs).

(if you spent 500 olives in a single transaction you can get one event ship during actual 8 year anniversary event) 

Otherwise no, play the SS4 as it is. Restorer on IJN SS (instead of often advised repairer) is usefull IMO. I prefer to survive an attack than trying to repair when forced to surface or below OH.

And besides that it seems that many players prefer IJN SS4 over SS5 

aa on a pca thats the fast way so u can lvl all of your crew at the same time

a ebb or pbb are good options to but the aa exp is just a bit better over all

 

The Hyuga EBB at lv 84 would be idea if you have gunners. You can get a lot of attack pretty fast. You can have 900SD on your SS4. I did until my crew got too heavy. I removed the rep from the gun position. Now I run at about 884SD, but I still OH at 40 Knots.

Assuming you keep as a SS BO.....

  Experience is dependent upon basically 3 factors:

-- How much attack you do

-- How well your team does

-- The level and displacement of the ships you sink       


The IJN SS4 is a BB killer.  You should do your best to dodge  ASW and sink as many BB5 or 6s that you can get to.  When your sailors get to the upper levels of the 20 level range you have, the HEI is an awesome and fun sub to have.  It hits so dang hard.  Learn your torpedo ranges, so you can get splash damage from the end of the line explode and use gradual firing.  I killed so many BBs with it that I stayed in it after getting the SS5 until my crew became too big.