understating how a hydrometer works with wine making?

I was wondering if anyone can help me in regrds to gravity readings with a hydrometer. Im currently in the process of fermenting some homemade mulberry wine..its been day 4 of the primary fermentation. On day two I did a SG reading and it read at 1.090 which I have read is meant to be a good reading for fruit wines. now at day 4 I did another reading and it now reads at 1.040..so the question I want answered is, if my first reading was at 1.090 and around 12% alcohol will this always remain or will it drop when the readin drops as my reading is lower from the first im just confused how it works as the instructions state that when your reading is at 1.000 then the primary fermentation is ready for secondry fermentation is this correct