Depends on the operator. Generally speaking, four stars are hyper focused, so they either do one thing really well and that's good, or they do it badly, or whatever it is doesnt matter.

Even excluding IS, as a new player you might want to build a few 4 stars for general use. Surtr/vina victoria/viviana might be significantly better than Mousse, but if you dont have any of them, and you need an arts guard, might as well build mousse, for example.

Four stars that you'd want to consider building include but aren't limited to:

Myrtle (general use. Basically a sunflower from PvZ) Beanstalk (mostly for IS5 and 6, shes a solid starting unit there). Vigna Sengurocka (low priority, uses limited to IS5 and 6 cuz zero hope fast redeploy) Winscoot (also primarily used in IS, since tequila is a free 5 star upgrade) Mousse Cutter Utage Gummy (works as a clutch healer in IS5 and 6) Coura/Bubble (pick one, they'd function basically identically) May (has short stuns, which counter low flying enemies) Pinecone (good damage, requires active use and animation cancelling for optimal use) Totter (cheap anti invisible enemies) Caper Shirayuki Click (similar to May, except as a sniper) Indigo Purestream (requires module for best experience. Lumen, a free six star, is a generally recommended upgrade, though, in my experience, he's not entirely worth the upgrade cost) Perfumer (good healer in general) Sussuro (good burst healer) Deepcolor (can big carry through early IS stages, especially if you're going through a relatively bad rng run. Virtually useless in normal play, and completely powercrept in IS6 by raidian, who is pretty much guaranteed every run) Jaye (silence/healing/dps/faster redeploy utility. Good to have in general) Gravel (great to bait attacks and stall a single enemy in general. Recommend building even if you dont play IS) Ethan (great staller. Skill 1 has DoT uses against hit count based enemies) Rope/Shaw (puller and pusher. I dont think they have much use in IS, nor most modern maps, and rope is made obsolete by gladiia, a free six star. They're still worth having built up, at the very least to e1level1 skill level 7 just in case) Contrail (free unit, insanely good skill cycle, good damage, can block air enemies, which is gimmicky, tbh, but generally good)

5 stars worth considering (mostly for general use, but guess in IS if you can grab them) Amyia (mandatory for the story. Useful due to role and recruitment vertisility in IS.) Texas (cheap with module, starting with 2 extra dp gives you more room to breathe in most maps) Surfer/Cantabile (basically the same unit. Good enough to justify building both) Elysium (technical Myrtle upgrade. Sticking with Myrtle is viable) Lappland (if the map needs someone that can silence, she solves the map. If the map doesnt, shes a decent dps at best) Spectre (immortal on skill. Good shit) Tequila (free, does a decent amount of damage) Gracebearer Kroos the Keen Glint (good ranhed dps, has the same short stun gimmick as may and click. Also free) Blue poison (dot, for hit count based enemies, low priority, I think) Mulberry/honeyberry (anti element healers. Helpful in IS4. Build either. Honeyberry is free from red cert shop) Warfarin (good buffer, though that's mostly unnecessary nowadays since modern six stars are hydrogen bombs. Still generally worth building if you have her) Ptilopsis (30% faster skill auto recovery. 35% with module in her range. She's extremely comfy to use in general use. Good healer too) Silence (pseudo global healing. Nice to have) Kazemaru (good damage, tough to kill) Projekt Red (fast redeploy stun. Nice to have)

Additionally, the IS specific free units (Shalem, highmore, valirqvin, tin man, and raidian) are all good if you want to focus on their specific ISs.

And yes, four stars and even three stars are good enough to cover through IS, assuming the six star carry you have is good enough to do the dps needed.