Friedman is definitely alive and still publishing, both as a researcher and popular author. I wouldn't hold my breath waiting for a new edition of
The Bible with Sources Revealed, though. His ongoing research uses the Documentary Hypothesis as a starting point, but I doubt that his conclusions will have changed all that much about the sources themselves. Also, for what it's worth, more than four sources were identified as far back as the late 1800s, but they're minor players and not discussed a whole lot. Most source identifications list all redactors as a single source, even though authors acknowledge multiple stages of redaction. Friedman simply splits out the several redactions into four individual sources, which brings us up to eight. He then identifies minor sources that seem to share no linguistic affinity with the other identified sources, making ten sources.
It's also worth noting that Friedman disagrees in a number of places with earlier scholars as well as his comtemporaries.
Joel Baden is a currently publishing scholar that disagrees with Friedman on some important points. His
The Composition of the Pentateuch: Renewing the Documentary Hypothesis is very informative, though I found it dry and difficult to read. The book lists for $50, but it's couponable at Kobo. I got it for $5 with a 90% off "we missed you" code.
I also recommend Julius Wellhausen's
Prolegomena to the History of Israel, which can be read for free at
Google Books and
Sacred Texts. Despite some 19th century turns of phrase and a
penchant for run-on sentences, the book is entertaining and relatively easy to read. There's a Project Gutenberg text, but it's terrible.
ETA: You also mentioned some interest in the construction of the Synoptic Gospels of the New Testament. Mark Goodacre's
The Synoptic Problem: A Way Through the Maze is available for free (and with his publisher's permission) as a scan from archive.org. I also recommend that one.