no mate, those sales ppl are full of bullshit, of course you can install 10.4.
you need to use disk utility to format/erase your hard drive (do it as mac extended (journalled and oh, i always liek to zero the data in the options, to ensure all data is completely wiped..). ok so once you've done that, power up the mac with the tiger installation cd in and your finger on the 'c' key.
you should get the usual screen saying to install and what hard drive you want it on and off you go..
:)