Will previous activities show on the new explore app update (4.0) ?

I just updated, and yes, you can see previous tracks activities in the map section. The standard topo map quality seems to have been downgraded (maybe its just different, I need a bit more time to play around with it), and you can only use offline maps with the $50 per year subscription, that's definitely an unwelcome change.

Yes your complete library will still be available in the new version of the app.

It's not true that you need an Outdoor Maps+ subscription to be able to use offline maps. The TopoActive map is available without a subscription and can be downloaded for offline use perfectly fine. Just tested this myself.

Thank you! This makes more sense that offline maps, like before, is available but premium/extra maps come at a cost. 

While TopoActive is available for me without subscription, that's the *only* thing available and it's not the same TopoActive:  Many lakes don't show names, others don't show until well zoomed.  Topo contour lines are only 100ft, the old was 20ft when zoomed in.

No free older satellite download anymore.  Also there is no availability for USGS quad maps at all anymore, some of which do have finer contours where it counts.

Also, there is no state-by-state load, only square sections.  There seems to be a bug in large square section loads.  A new iPhone and iPad had no trouble loading 2.6GB of the western US.  My older iPhone tried for hours: it'd get up to just over 1GB and go retrograde - the load size started going back down.  I'd check and it'd have 900MB then 800MB then 500MB, then later 700MB again.  I sat for awhile and saw it going down from 1.21GB to 1.1 to 1.0 to 950MB  :-/

It's early days for this new version but with all the bug issues for the Fenix series and who-knows-when this stuff will be addressed, I may be glad that Apple has added trails and download ability - may be time to add an Apple Ultra and leave the Fenix only for the odd backpacking trip.

The paid subscription thing seems to be a bug as far as I can tell, so that's good at least! But I haven't really used Explore to create many hiking plans for a while now, I'm doing that outside of the Garmin ecosystem and just uploading them to my Epix through Explore. I had hopes Explore was going to be an improvement over Connect in that regard, but let's see how it iterates over the next while. It'd have to get pretty bad before going to Apple...