Baby Development by Week Question

I found out I'm pregnant for the first time after 18 months TTC and IVF on Wednesday!! L I understand how the weeks work.. I think. I'm 4+1 today, so I believe that means I'm in my 5th week. My question is, how do you read the apps and books that share baby development by week? Meaning if I want to know what happening this week with my baby do I look at week 4 or week 5? This has always been a confusion of mine. Thanks for any and all help!!