lost audiobook on iphone 4

When there are audiobooks on your phone you should get an Audiobooks option under the More button at the bottom of the Music app.

Note that audiobooks are currently a one-time only download from the store, so you may want to copy it over to your computer's iTunes via File > Transfer Purchases (File > Devices > Transfer Purchases if you have iTunes 11 on your computer) so that you can take a backup copy of it - it whould copy over to the Books part of your ITunes library.