Hi Salsa,
we just introduced it straight away, it was not a problem for her. The reason we did this at 4 months is because she was taking a long time to settle in her bassinette ( we would rock her etc) and was not sleeping very well during the day. Also, it took me a while to convince DH that we needed yet another expensive baby item
I think they are great and will use it from day one for any other babies we have. HTH