Wikipedia says “the average life expectancy for male indoor cats at birth is around 12 to 14 years, with females usually living a year or two longer”. I know, not the best source, but I’m too lazy to find a better one. My point is that Stella isn’t elderly - she has a lot of purrs and cuddles left in her.
If I were you, I’d put down the money to get any tests she needs to determine whether the cancer has spread, and then google the crap out of success rates for treating the type of cancer she has in that location. I wouldn’t trust any one vet’s opinion who could potentially be treating her, because I’m sure there are vets out there who will be overly optimistic to get that 4.3k out of you. And then you can make an informed decision.
The only useful thing I have to add is that the cost of vet care seems to vary with the cost of living in your area (for routine shots and wellness checks at least) - so if you live in a city, you might want to price out vets in the suburbs. Or if you live in the suburbs, you might want to price out vets in the opposite direction of the way everyone drives to work in the morning.
IMHO saving the life of a beloved pet is one of those situations where racking up some debt is justifiable. And with each temp job you’re gaining experience and contacts that increase your chances of finding something permanent. But then only you know the details of your life to judge whether you can take that hit.
If you don’t already have one (and if there’s anything available in your area) could you find a part-time second job? Even if you’re only serving coffee / serving food / working retail every Sunday, you’ll have your foot in the door - so you’ll be in a position to ask for any available hours if you find yourself between temp jobs again. And that extra x a month could go towards your payment plan with the vet, or paying off the credit card bill from Stella’s treatment.
Is there some kind of kickstarter-like site for charitable causes out there? (DIY alternate: website and paypal account.) You could put together some heartwarming videos of your little friend, offer to refund money if you don’t raise enough, and cross your fingers that some wealthy animal lover is in the right place at the right time. It’s a long shot, but at least you’ll know you tried.
Maybe I’m a hopeless optimist, but perhaps in a few years you’ll be thanking Stella for giving you the extra motivation to go all out on your search for more permanent and lucrative employment - I’m certain that after 5 years, I’d be phoning it in, but hopefully you’re made of sterner stuff than I am.