Stats of 5* at 3.70 are generally slightly lacking compared to the average maxed 4*, but on the other hand a 5* special is typically superior to a 4* one, too. I agree with the general sentiment of what’s already been said, but for me it’s really very much a case by case decision.
Stats at 3.70 for those heroes you mentioned:
My thoughts by color (again considering only the heroes you mention):
Red: Tyr has serviceable stats at 3.70, nothing particularly overwhelming or underwhelming. His special skill is fast, and unlike any 4*. He looks useful at this level. Kelile would do more direct damage, but is lacking the “special” parts of Tyr’s special - which is more useful depends on the circumstance. Scarlett also does more overall damage, but less focused (target + neighbors), and gives -34% attack to 3 enemies. Her attack stat is really high for a 4*, which is good for tile damage (she might do almost as much damage with her special to her “main” target compared to 3.70 Tyr even), but on the other hand she’s one of the squishiest defensively. I see use for all 3 of these, depending on who/what you’re facing. I would say Tyr would be the strongest choice by far defensively, Scarlett is strongest offensively, and Kelile is the best all-rounder. Kind of a tough choice, rationally I would’ve picked Kelile first because of general usefulness (she’s actually super similar to Marjana at 3.70) but I do like the other 2 also.
Green: Telly leans a little stronger defensively with her stats, so decently survivable compared to most maxed 4*. Sadly I don’t have her so no direct experience, but I expect she’s still a really strong tank at this level among 4* and other 5* at 3.70, given how devastating she is at max. Brynhild is a good one too I think, though limited as general healer since she heals only 3. Caedmon is an excellent all rounder, a fast sniper that dispells buffs from all enemies, which can be extremely valuable. Kind of a tough choice, I think if I had to choose which to go with first, I’d start with Caedmon, and then Telly.
Blue: As has been pointed out, Isarnia is definitely squishy at 3.70, which is not a great combination with slow speed. However, all 3 of your blues are entirely different heroes that you would use for entirely different purposes. Issy at 3.70 can still be quite good against relatively low level Titans for one (especially against red ones obviously), the -44% defense will make a big difference there. With the +30% attack and defense boost, Kiril is a fantastic healer, one that will still be useful basically forever even when you’ve got plenty of 5* maxed. And Sonya is basically a blue version of Caedmon, but since you have her costume, she’d get an extra nod. The order I would go with for blue is Kiril first unless you already have multiple good healer options, Sonya second and Issy third.
Yellow: Malosi’s stats at 3.70 are also serviceable, not remarkable really. He is very fast of course, so that’s a big plus. He won’t do huge damage at 3.70, but the block on status effects is definitely useful. I don’t have actual experiencIe with him either, but he seems quite useful to me even at the 3.70 level - though his usefulness is somewhat limited to opponents that cast status effects. Wu Kong… some people love him, some people hate him. Unless you have a Ranvir or Tarlak or Miki, I think you will want him for the huge boost he can give to your Titan damage. He’s can also be good on offense in war/raids, in the right situation. With neither of these being real all-rounders, which to choose first really depends on what’s more important to you. But for the Titan damage alone, my personal choice would be Wu first.