how to play scales well

Think before you play and listen while you are playing.
There should be a general shape (e.g. crescendo and decresc)

The only way to play well is to practise more and control you muscle strength.

For volume control, usually when you playing up, you should do it in cresendo gradually until the last octaves and diminuando (forgot the spelling) when going down. (I think this is "just on cresendo in four octaves" in your words.)

Since the volume of lower keys usually higher (means louder) then higher keys, so the key is to control your lower keys volume and use more force to play the high key (as higher keys' volume is lower) in order to do the overall cresendo effect. Same case applies when going down as you need to do an overall dim. effect.

Besides, when you listen to you scale, the sound should be smooth.

Even if you play staccato scale, same thing applies.

Good luck!