Randomly but Evenly Distributing Tokens on a Board
The more dice you use to generate a single random value, the closer each random value will be to a certain average. You can see this happening in Catan: rolling two dice results in a lot more sevens than twos.



If you do this with 4 dice, almost everything will be centered around the 14th coordinate. If you don't mind some normalization, you could use 2d12 instead of 4d6.

You could give the players some say in where the pieces are placed. There are many ways in which you could do this, either before or after the dice are rolled.

If you want 'complete' randomness with dice, you'll need a 1d10 and a 1d20, or two decks of numbered cards.