Place dynamic marking under bar line (move dynamics horizontally)

I want to create a crescendo that extends over two measures.

The following code

\relative c'
{
    c4\pp\< c4 c4 c4 | c1\ff
}

compiles to

which is almost what I want. However, I would like the ff markings to appear further to the right, directly under the very next bar line, to indicate that the last whole tone should have a gradual increase in volume as well.

How can I achieve this?