Well, the 5208 comes with 10 zones by default, & can be expanded to a total of 30 zones with zone expander modules. The 4006 comes with 5 zones by default, but can be expanded to a total of 10. Both have built-in dialers & 4 NACs. Not sure about the 4006’s programming, but I know the 5208’s programming is incredibly flexible; it comes with a number of zone types, & custom coding options can also be programmed using the software, among other features. The 5208 can be programmed using either the keypad or the software; the 4006 can only be programmed via the keypad as the software for it is unavailable to non-technicians.
I guess the MS-4 is a decent panel at best, but like I said the downsides are only 4 zones (which usually aren’t a problem for collectors starting out like you, but still), 2 NACs, FWR output, & non-programmable relays, compared to other fairly small panels, some of which have more or better than that.