Since no conditions are there for white balls,
All the 7 black balls can be arranged in one way.
White balls are to be placed in between the black balls.
There are 8 places for white balls and 4 white balls are there.
Any 4 places are to be selected out of 8 places in $^8C_4$ ways.
$\therefore $ The required no. of arrangements = $^8C_4\times 1$