Why not go to five steps?  It's basically the same as four, but with one tiny step with the left foot (right hander) at the start. 

Ball speed is easier attained with a free arm swing and slightly faster feet. There are tons of great bowlers who take 5 steps, and lots who take 4, but are there many out there who take 6?  Just curious...

Increasing foot speed doesn't always increase ball speed, sometimes it can actually DEcrease it if you don't pay attention to your timing. 

Go to a 5 step delivery. I have done the traditional 4-step approach my entire life until about 3 weeks ago. I got aggravated with some timing and speed issues and figured why not. I actually watched a Chris Barnes video on youtube about switching from the 4 step to 5 step and then attempted to implement one night in league.

Really it is all about that first step without swinging the ball. Focus on step -> step/push away -> 3 -> 4 -> slide.

After about 4 frames it became habit and I haven't looked back since as I have actually increased my ball speed and improved my timing greatly.

There is no single method that will work with everyone, that is for certain.  To increase ball speed you need to either, 1)  throw it harder or 2) increase momentum on your approach.

Throwing it harder can cause trouble with consistency and shot repetition, but if you free up the swing, then for most people, the feet want to go a little faster.  Let them. 

Timing gets screwed up when you try to keep the feet slower than the arm swing dictates.  Look at all the women who can generate 18-19 mph with seemingly no effort.  It's obviously not a muscle thing.  It's letting gravity and natural momentum help you.  You may be surprised as your accuracy picks up in direct proportion, too.  Only you can decide how many steps will be best for you.  Good luck.