You can download these if you like - Foclóir
http://www.seomraranga.com/tag/focloir
Leabhair labhartha
http://www.scoilnet.ie/leabhair.shtml

Here's another idea for you and your four year-old - you can print out some of the shorter poems that you can see here and make learning cards with them, not forgetting the pictures of course.
Rainn agus amhráin
http://resources.teachnet.ie/clane/2008/index.html
and there's - Snappy First Words in Irish (Templar Publications 2008) by Libby Hamilton and Derek Matthews ISBN 978 1 8401 1688 5 (it's available from Amazon)