Concatenate an underscore with a string in Postgresql

I'm trying to build up a string that will look like this in Postgres 9.3.

2015_2_23_10

It's the current year, month, day, and hour, separated by underscores.

Here's the query that creates the string

select 
    cast(extract(year from now()) as text) || '_' ||
    cast(extract(month from now()) as text) || '_' ||
    cast(extract(day from now()) as text) || '_' ||
    cast(extract(hour from now()) as text);

However, the underscores get turned into spaces. I also tried prefacing the underscores with an E, like so ...|| E'_' || ..., but that didn't work either.