irf530 Id calculation
This is a bad way to control LEDs. For a start you have them all in parallel - have you done no research at all about LEDs? LEDs need individual current limiting/sharing resistors when placed in parallel or one of them hogs all the current and blows up after a short while then, the current that was to be shared among 4 LEDs is now inflicted on three and, the one that has a slightly lower forward volt drop, takes most of that current and blows.

You can see where this is going  - basically after a few minutes or seconds you have no more working LEDs.

You have a 12 V supply and four LEDs which I assume to be standard 2V, 20mA types. So, put them all in series and calculate a current limiting resistor to go in series with them.

If you want to control LED current to control brightness then try googling current sources like this one: -



Varying R2 can give you some control on brightness. Of course you could consider a more elegant solution involving a proper LED control chip: -