I am only going to compare base games together and what I know to be fair (can't compare a complete game with EP, SP or DLC with a base game that is brand new and not out yet). Which unfortunately doesn't leave much pros as the stuff I did like about Sims 3 wasn't in base game, but came later.
Sims 3
Pros:
-Story Progress
-I liked the age slider as I could set how long the sims lived for and for how long until they aged up the next age. Hopefully Sims 4 will have that as well.
-Color wheel was convenient, even though I disliked the rest of cast and hardly used it (can also do the same without using .
-Can take babies off the lots (in the neighbor). Lot of items didn't come until later with EP or DLC, so they don't count to be fair.
-If the game was stable enough and you are builder, that game is for you I suppose.
-Toddlers if you are actually into that.
-Open world if that is what you are into.
-Pools (though lack of items in base).
-Ghost I believes (though unplayable at the time and unsure if we will actually have ghost that show up eventually or not in Sims 4 base game. Could be like Sims 2, where a sim died, ghost didn't show up until later at night, as long as you have the grave on the lot).
Cons:
-Ghost weren't even scary or cool like in sims 2.
-Story progress didn't work properly half the time without mods and I am sure we will get mods to help with that,
-Open world too buggy, took to long for sims to travel and often freezed and crashed.
-Sims seemed empty to me and lacking in lot of the interactions.
-Lot of items were missing in base game that was in sims 2.
-Sims 3 randomize sims often turned out horrible.
-Ugly Sims in general
-Burrito babies.
-The exchange was a nightmare and modding for the game was hard.
Sims 4
Pros:
-Easier to mod and make cc I heard, already got some neat stuff with just the demo out.
-Gallery makes it so much easier to sims and houses in game without logging off and hopefully easier to take out of game.
-Search buttons, categories and subcategories to make things faster and easier to find.
-Apparently you can order different types of pizza.
-No need for a lot of sliders or waiting for them as can now just push and pull to edit sims
-Actual ethnic sims with many different skin tone choices.
-Getting a lot of items and skills in base game, stuff that was taken out of Sims 3 base game. We are getting bars, clubs (had to wait for Night Life in Sims 3), new careers, etc....even bake birthday cake (could only do that with that Bakers set that was a DLC in Sims 3). Also wedding cakes and arches (wasn't in Sims 3 until Generations or with wedding set DLC). Science skill didn't come until University Life. Chemistry set not until generations, etc....
-more interactions with babies, so they aren't completely boring to care for, they got limbs again and even have the option of breastfeeding
-No more parents keep moving the babies to different cribs as they stay in one crib or putting them on the floor all the time.
-Supposed to be less serious bugs like major freezes and runs smoother.
-Cupcake machine, enough said.
-Pregnancy lengths can vary from 3-4 days, no forced maternity level, can take pregnancy test and can ask about due date. Plus any clothing can be worn while pregnant, so more more ugly or storage of clothing.
Cons:
-Currently no pools.
-More loading screens, thought they aren't that long at the moment and supposed to be not for every lot in the neighborhood your sim is in.
-Can only interact and pick up babies near the crib.
-Eh currently no toddlers (though for me, isn't really all that upsetting and they are planned to be back in soon as they are coded in the game, just not finished).
Tried to be fair, but not much of fan of Sims 3.