I too returned after 4 years absence and wholly molly mate, the game IS HUGE!!!!! SO much has changed and there is SO much more to do.
I had 4 level 50 toons but I actually started all over just to refresh myself with the game as so much has changed. Get the ESO plus if you plan on staying, and why wouldn't you....its a a blessing for sure and you get ALL the DLC conetent. There is a lot to catch up on and even more coming...DRAGONS and Necros! So yea, you picked a good time to come back and you wont be sorry really!!!
Again, I suggest starting over and reacquaint yourself. AS for builds, best to just wait til 50 and then start looking as most builds out there are aimed at end game or trial content. Some do dungeons as well. Personally I suggest https://www.xynodegaming.com/...for builds and he also has an awesome Dungeon guide series. His build vids are very thorough and explain everything. For some that can be too much so others might suggest a few other "content creators", there are quite a few so just check em out and see which you like.
Best to just get stuck in and see how you go! I am on PC/NA if you are and have questions. Enjoy and welcome back!!!

Welcome back!

A lot has changed, and I would highly recommend considering making a new character to get the full scope. But here's the main things that I personally think stand out. I don't remember exactly where the game left off 4 years ago, so some of this might be old news to you, but here goes

Basics:
First off, racial passives have all changed, so depending on how much you put stock in these I'd suggest checking them out before possibly starting new characters. They have also implemented a leveling guide and a level reward system - meaning you can get recommendations for skills and morphs when leveling up, as well as getting rewards for each level up to 50 - some of the ones worth mentioning are a mount, costume, pet, EXP scrolls and a bunch of consumables.

Crafting and such:
Housing and furniture crafting is now a thing. Houses can be bought in all zones (and you are able to get a free inn room in the starting zones of each faction), and furniture can be bought and crafted. There is now also Jewelry crafting with the release of Summerset. Motifs are now converted into "outfit styles", which can be used at the Outfitting stations (formerly Dye stations). Here you can not only dye your outfit, but also change the style of each piece of armor AND weapon/shield, for a Gold fee. Changing dyes of non-changed armor is still free though

Class and skills:
They have added a new class (available via Morrowind pack or as a stand alone purchase in the Crown Store). Classes and skills have been tweaked quite a fair bit over the years, so take time to read through your skill/morph descriptions. It may be worth considering redistributing your skills from scratch if you are starting back up on an existing character.

Subscription:
It is no longer mandatory, but is still an option. You get EXP boost, Gold drop boost, Crafting boosts, Crowns every month, and a bottomless crafting bag - which is a great convenience, but not a must. You also get access to ALL DLC zones and dungeons (except the newest chapters, i.e. Summerset), so it's a great way to try out content.

The world:
Since One Tamriel, all factions now share the world together - which means you are no longer bound by faction in regards to PvE content - you can now play with anyone from any faction. There have, of course, been added a lot of new DLC zones - but don't feel rushed to try these out. They are all pretty great, but most aren't very big and it's good to keep some of the new stuff for after you have reacquainted yourself with the base game. Also, a lot of the story/characters continue on in the DLC zones, so doing the base game zones first gives a better sense of immersion in the future.

And finally, take time you reacquaint yourself with both your Journal/Achievements and your Collections tab - and the general UI. I feel this will give you a good indication of what has been added to the game. There is also zone guides now for each zone/map and under the Help tab in the UI you can get most of your questions answered at your own pace.

That was a mouthful! Hope it all makes sense and helps just a tiny bit. There are of course a myriad of changes that I haven't mentioned or remembered, but hope this covers some of it. Feel free to hit me up with any other questions, and if you are on PC/EU I can even help in-game

Happy hunting traveler!