If I want to get into fighting games having played only casually before what game should I get? I liked SFIV
SFV still has mixed opinions, but I'd check it out with SFV Champion Edition offering almost all the content for a low price. They did just announce five more characters though.
Mortal Kombat 11 just got its Aftermath expansion, so it's full priced again at $60, but there's a lot of content.
Samurai Shodown is a pretty beginner-friendly game in that it's not combo-heavy and has more of an old-school feel. You can often find it cheap (physical) on PS4 and Xbone, although it does have two season passes right now.
Granblue Fantasy Versus is pretty easy to pick up and play and is a little less crazy than Guilty Gear and BlazBlue. Dragonball FighterZ is also easy to pick up and play, but it's pretty chaotic. Unless you want something like Marvel Vs. Capcom, I'd stay away. Same with BlazBlue Crosstag Battle.
Undernight In-Birth is another good Street Fighter-esque fighting game.
Tekken 7 is still going strong, although you'll have to get a couple of season passes to get all the characters. Ditto on Soul Calibur VI.
Would recommend checking out Dead Or Alive 6, because there's a free-to-play Core Fighters version and if you like it, you can also just buy individual characters and just pay a couple of bucks to have access to who you'd want to use.
If you want good old-school offerings, check out the Street Fighter 30th Anniversary and the upcoming Samurai Shodown Collection. The latter will be out next week for free on the Epic Game Store on PC.