Welcome to this forum thisengineburns. You will be delighted with all the wonderfull information around early development. At four month it is an excellent time to start. You can read about YBCR, LR and tweddlewink on just typing on the serach space.
Do not forget that all this activities take only a few minutes and you have to watch your baby's reaction. If you see he is not enyoing it, well stop and try again some other day. Also play music. I bought 'Bethoven's Wig' and 'Themes to remember' which i learn about reading smoe threads here. Both are very good and my grandson like them.
Oh, one important thing at this age is the ability to learn other languages. Try to get videos or music so that hi ear can get use to the different sounds around the world. If you or anyone in the family speaks more than one language, it is an excellent opportunity to give your child that preciopus gift of being bilingual, trilingual or more.
Hope you enjoy this travel with your baby. And again, welcome.
My wife and her family are fluent in another language so he gets both every day.
Ordered the 1st Twiddlewinks CD and dl'd the phonics touch chart and flash cards. He watches YBCR every day (starting disc 2) and also Baby Signing Time. Loves the songs.