The main problem is, I simply cannot afford them ( I would have to use an expensive credit card)
My advice is, enjoy what you have, but stay away from "credit". If you can afford it in pure cash that's a totally different thing.
But, is this 40% off offer to good to "run away from"?
Not really.
Someone should knock on my door and say enough is enough, use the B5s ;-(
For surrounds? Look through your collection and evaluate how much your surrounds are actually contributing. I suspect not much.
"Is PSB Imagine mini a significantly better speaker than PSB Image B5/B4?"
I doubt it. Even if it is, it's not worth eating white bread for. Stick to 100% Whole Grain.
I recently upgraded from the very nice EMP E55Ti speakers to the
amazing Philharmonic speakers. Were they worth paying what I paid? Yes. But it's not like my life changed. They're just speakers. White bread is gross.
What I mean is, when I first upgraded from "whatever" to the EMPs, it was a night and day difference that actually gives you a feeling of euphoria consistently. You really were hearing things on a regular basis in unexpected places that "frustratingly" was masked on everyday speakers. Sure I already knew some of this from headphones, but what stood out wasn't the stuff I sat down and listened to critically, but rather everyday stuff - video games, movie soundtracks, sports, and anime. The critical listening stuff is fun, but in its own way it's almost tedious because you're becoming focused on the equipment, whereas in my opinion the equipment should dissappear.
In my opinion, yes better equipment sounds better, and you can no doubt hear it - but it doesn't "dissappear" any better - you may not notice right away if someone swapped it out.
Even if there are details being masked by medium level equipment, they may not be "worth the effort", so to speak. it's like, 90% of the time, the medium level speakers are good enough for the medium level source material we subject ourselves to. The better speakers really shine on the other 10%, but it's not a life altering difference the way the jump from poor-to-medium is. It's more like

rather than
So my question to you is - what are you chasing.
Most of us here like hi fi / HT as a hobby, but we're pretty much pretty happy with what we already have. Upgraditis is not necessary - it's just our irrationality speaking for us.
Some of us even have more systems than we can actually listen to.
If you're chasing something better, ask yourself if it's because you're currently dissatisfied, or if it's because you dream of somethin magic. That something magic probably won't happen "often" enough to justify "
sacrifice".
To repeat myself again, stick with monopoles for multichannel music and avoid bipoles. You don't need them.
Essentially, if you look at any multichannel mixing desk / room, you'll see ordinary speakers as surrounds.