How to act on google dancing

i was rank 3 yesterday, now on page 4 for main kw.

what to do?

It is up to you to make the final decision. Are you still making enough money at 4 to be happy? If not, blast it with high PR do follow backlinks. You take the risk that Google may mess with your ranking even more though. If you are making enough money to be happy, just let the website be and don't touch it for a bit longer.

Just keep building links you will retain your position soon

Just keeping building backlinks at a steady speed - then just wait and see.
Most of my sites bump a few listings from week to week.