Yes, for CCNP Security you need all four exam and also have the prerequisite which is CCNA Security. Some time ago, the older CCNP Security exams counted towards the "new" certification, but that's not the case any more.
I would use the following order when studying and writing the exams:
- SENNS
- SIMOS
- SISAS
- SITCS
I have no info at all about this, but I would expect that sooner or later at least the SITCS will arrive in a new version. This course (and exam) still focuses on legacy technology like Cisco IPS and ASA CX which is not very relevant any more.
Have fun studying for the CCNP Security!