To solve this equation for x, we must first simplify the equation by expanding the terms.

300 - x + 1/2 * (500 - 300 + x) + 300 - x = 4x

First, simplify the term inside the parentheses:

300 - x + 1/2 * (200 + x) + 300 - x = 4x
300 - x + 100 + 1/2x + 300 - x = 4x

Now combine like terms:

700 - 2x + 1/2x = 4x
700 - 3/2x = 4x

Now isolate the variable x:

700 = 7/2x
x = 1400 / 7
x = 200

Therefore, the solution to the equation is x = 200.