If you are good at making money, upgrading a relic is only a matter of time.
I think people look at the amount of gil it takes and think it's harder than it is.
I am not saying it is easy but when I started I was a bit overwhelmed by how much gil it was goin to take and after maybe 7 months or so its not so bad.
You just need to figure out what to do to make gil. You need to do multiple things to decide what works best for you.
I have done everything from farming misteltoe, wisteria lumber, NMs like Ga'Bhu Unvanquished, to selling znm stuff.
You need to spend at least 75% of all your game time farming/crafting/whatever it is that you do to make gil. You really have to put your heart into it.
Farming is different for everyone. Progress is different for everyone. Find something that is worth your time that allows you make steady progress that you are comfortable with.
Finding something that is also fun as well as profitable is a plus.
I'm going to say there is no relic weapon worth the 40-50mil it costs to upgrade stage 4 that would be worth leaving at stage4.
Level a relic for the love of your job as well as take pride in helping your friends/group/LS as much as possible.
A stage 4 relic wouldn't be much use to anyone. including you.
I hope this helps
Good luck in your journey my friend.