Continuous Probability functions

http://myasdcommunity.com/continuous.jpeg

(I keep getting not a valid image file, sorry, so couldn't display :S...)

I've managed the first two parts of the question. How ever i was a bit confused as for how the second question the solutions managed to obtain:

For the integral of the function.

Thus for the third part of the question, i am confused:

It would make sense to have to work out P(X >= 4) which would be

1 - P(X <= 3)

However they sub in 4 into an equation and get:

And i am very confused as to where this comes from :S

Thank-you.