I use a Newsletter plugin for IPB 3.4, to get members alerted about new contents in the forums (on www.99nicu.org).

Will there be a built-in Newsletter feature in IPB4, in which content from forums, blogs etc can be easily added and distributed to all members?

Or will this feature still rely on third.party solutions?

You are able to send Mass-eMail to Members - but you are not able to add Content from your community automatically or by Drag and Drop,etc out of the box - you just enter your text in the editor like posting a new topic.