As Russ said, It's not a vector pattern anymore , so search for tracing bitmap or perhaps a trace tool that wil convert the pdf into a .cmx and that you can use.
I have Corel myself and there is a grafic tool called corel trace, but you started the topic with corel draw X4 and now it's CS4.
I don't know CS4, but can you import the pdf in CS4?
if so then press F1 for help and search for layers.
make a second layer on top of the original and write your letters on that layer.
Fill the letters with (white) paint and you have you new pattern.
else: you could try to convert the pdf :
http://www.freepdfconvert.com/