Synergy
Depends on your definition of a bad 5* hero.
To illustrate my point, many people prefer 4* Boldtusk to 4* Scarlett, but a Wu Kong cascade squad works better with Monk Wu Kong, Cleric Rigard, Druid Melendor, Barbarian Grimm and Rogue Scarlett. Though Boldtusk can substitute for Melendor if you are missing Melendor.
It is all about synergy/ heroes that work well together for your first rainbow 4* 4.1 team and rainbow 5* 3.1 team.
Depending on your other 5* heroes, Druid Guardian Owl may be perfect. Really unlikely, but Devs like adding new 5* heroes.
Aside, you have the heroes, I would strongly advise taking to 4*+19 Wu Kong, Rigard, Melendor, Grimm, Scarlett. They work really really well together. Barbarian Grimm against titans is one of the few places Wound is actually useful.
3* / 4* Ascension items, Hero XP and emblems
You can do most things in Empires with a Level 17 Forge and any rainbow 4* 3.60 team.
4* 3.60 heroes cost 24% of the Hero XP of a 5* 4.80 and zero of the hard to get 3* ascension items needed.
You can do anything in Empires with a Level 17 Forge and a good rainbow 4* 4.70 team.
4* 4.70 heroes cost 48% of the Hero XP of a 5* 4.80 and 50% of the hard to get 3* ascension items needed.
Theoretically you can do anything in Empires with a Level 5 Forge and a good rainbow 4*+19 team.
4*+19 heroes cost 35% of the emblems of a 5*+19 hero. And you can later reset the 4*+19 if you get a 5* equivalent. Example Rigard 4*+19 and Ariel 5* 4.80.
Though personally I would keep both if the 4*+19 is good.
My personal opinion
I would not level a 5* hero to 5* 3.1 until I have a good rainbow 5* 2.60 team.
There are exceptions I would take to 5* 3.70 like Joon, Alberich, Natalya, etc. that have insanely useful specials even at 5* 3.70.
Edit:
My wife uses Cyprian on her attack team ( she can get to #30 on the Global Top 100 list ) but he takes lots of practice to use well on the attack. Her defense team is at 5* heroes.
I find Tiburtus 4* 3.60 works well when I need Ramming Pulverizer versus a yellow Boss/ Titan.