its actually not wrong to invest on a 4* unit, my first 2 main dps was keqing and xiangling, yes...a xiangling, she was my 2nd character i invested to lvl70, normal atk & pyronado to talent6, she carried my 2nd team to floor10 abyss. I handed her gears down to Zhongli after i got him, its not a dead end.
Most 4* are actually really really great endgame units, razor, beido, ninguang, chongyun, bennett, xingqiu, sucrose....they are top tier even with 5* into consideration.
You need to plan your team BEFORE investment. not spreading your current limited supply evenly across your current hero pool, theres a difference.
Supports in general dont need major investment to do their job either, my barb was on 4* exile x scholars, not even upgraded +0, ppl are shittin on her mainly cuz shes "free" and gives wet status, but she helps the most in almost all my fights with thrilling tales. Wet status cleanse pyro aura effects too.
I ran floor 9-2nd team with a lvl79 keqing, lvl60 barb, lvl8 kaeya, lvl10 amber. its 8/9 star completion. the idea of support is to use their elemental reactions and skills to counter shield and gameplay mechanics allowing your MAIN dps to do most of the job later on. Barb works EXCEPTIONALLY well breaking pyro shields.
Domains are more unforgivable in certain sense, ESPECIALLY noblesse farm, a team mate agro-ing all 3 of the mages would put team at risk if they are doing the pyro & cryo mage clear 1st strat, cuz hydro mage is out of the equation of the first half duration of the fight, thats HUGE, cuz you dont want to get frozen and bubbled most of the time focusing on pyro mage.
I could go on and on breaking down domain fights, but really you must at least understand how certain decision and strat you applied to your gameplay especially in coop makes MAJOR difference.
I could solo Childe, but coop makes it 2x harder depending on your partners unit if they lack coordination and sense to dodge debuff or mobbing.
AR 49 here thinking of upping Amber next lol. Just because fuck floor 12. And I have max built Razor and Xiangling. Nothing wrong with 4*s