To clarify your questions
-Gear still keeps inherent armor value which chest has the highest armor value followed by pants, helm, glove and boots. Weapon, glove and amulet now can roll with maximum life.
The lower armor cap is a buff not nerf, actually is not much difference for most class as if you have juggernaut aspect usually you can get to 16k above which is the cap in S3 for lvl 199 monster. In S4 armor cap it will be down to 9230, which gives you an extra option of not using juggernaut if want a faster evade cool down like sorcerer TP enchantment or Barb metamorphosis. Another benefit is you can roll max life on gears instead of armor as you will have more than enough. Armor and maximum life can be rolled on more gear in S4 including glove and its max value is higher as legendary gear comes with 3 affixes max. The only downside of near gear is DR affixes are gone except Unique like Tibult Will and Shakro
-No changes to Uber Unique but you MUST UPGRADE and add Socket to them before S4 starts as Upgrade will be replaced with MasterWorking which you can’t do it on legacy gear including Uber. As Uniques include Uber cant be Temper, so it has lower stat and is worse than Legendary gear for some cases including Grandfather due to Legendary gear can be Tempered and has a higher maximum Affix roll.
-Any new Unique or Uber Unique you acquire again in S4 you can MasterWork them to increase its stat, also some Unique may drop with Greater Affix which has 25% higher affix max value, if you lucky to get on on Uber. Nevertheless I don’t recommend farm Uber again too hard if you already has one as Blizzard may further improve Unique next season. I am not sure those future changes will apply to our Uber retrospectively so you don’t need to do another Uber farm next season. But I think most likely future changes will apply to Uber you get in S4 as the changes in Ring of Starless Sky and Melted Heart of Selig affect the older version. But it will not affect the legacy Uber prior to S4. Some Uber Unique still better than Legendary gear due to its Unique effect like Shakro, Ring of Starless Sky and Melted Heart of Selig (for Barb) even its affix are weaker and cant use Tempering.
-Most things are improved actually including the paragon board, just need to tweak them. You can just copy the build shared by streamers so save your time or you can plan it yourself. You can refer to Maxroll as they have a very detailed build planner for each build or you can just search youtube and follow your favourite streamer for the class.
-No point in wiping your current character, as they were not changed. Only the gear changed and also some skill tree or paragon changed. Most classes received buff except Barbarian which nerfed in many areas due to they were overpowered for 3 seasons.
Season 4 is best for Eternal due to no unfair Seasonal Buff in the Seasonal realm. So is time for you to play all the classes and acquire new gear. Will see what happened to S5, as Last Epoch also offers the same content between the Eternal and Seasonal realm. As Blizzard is currently copying features from other ARPG so I won’t be surprise if they copy this feature from Last Epoch too. Diablo Immortal also let the player keep their progress in the new season, but it is a Pay2Win game for PVP which is another topic.
Another improvement is Aspect can store in your codex. So you can salvage all your legendary gear which you keep for Aspect, this can free up your stash. I assume all the legendary aspect you extracted previously will gone and directly upgrade your codex to the max value you own. Do remember login each character if you store the codex in their stash.