Scribus - up to version 1.5.8 - does not contain any tools for imposition, as far as I know.
Typically the work flow is that first you export your "raw" PDF pages from Scribus and then use another tool to arrange as needed.
If you search for imposition and for PDF, you will find many free tools and even some that go with the open source idea, like Scribus. Many of the "easy" ones, will not be able to handle bleed correctly.
Personally, I have had frustrating times, trying to do it with free Adobe Reader, as there is some size-change happening, and out of my control.
A colleague in our project likes the new "MS print to PDF" on her Windows computer. I do not know details, but normally she gets all her booklets done fine, so it must be working. This is just in-house printing on a laser on A4 sheets, so no bleeds, no final cutting involved. You could use this to make sample-printouts in your own office to show around for feedback and for final proof-reading.
I have an old licence for Adobe Acrobat and that can do imposition, but is expensive and I do not know recent versions and their features.
What I ended using, and really like a lot is MONTAX imposer: It is commercial and there is a limited free version, which will only work with A4 sheets max. Since you need a 2mm bleed, you will exeed the A4 sheet limit and would need to spend 95 US$. That sounds a lot, when coming from Scribus. If you are interested, you could reduce your pages in Scribus in a dummy-copy and just try out the free version for a few minutes.
When you first look at MONTAX imposer, it might seem to be an overkill. What some call a steep learning curve. But if you do your flyer more than once, you might like it.
I like it a lot, it even gives me useful feedback when I overlook something. And there are plenty inbuilt samples to guide me; I even learn the English vocabulary about imposing from this tool, that does nothing but.
The vendor is kind and helped me, even as a non-paying customer, when I first started. If I ever had a bigger project, I would spend some and get the fitting package.
Side-track-idea: our online print shops allow us to select a "product" (like your four in on flyer) and then upload single pages and they give us very clear instructions on how to name each page so that everything will end up where it needs to be.
I am not saying "change your printer" but imposition is something that falls between the layout and the physical printing and is often done at the printer, even if you do some "virtual imposition" at your desk. Only the print-shop can know the real size of the sheets (or roll) of paper and so they always have to do some final imposition anyway.
I am in no way afiliated, just sharing something I appreciate with each newsletter we are putting out (hundreds of semi-automatic impositions after using the ScribusGenerator script).
The products page of MONTAX imposer
Finally, I am just sharing a personal like about one tool. I am not claiming it is the best solution for the question. But it can most certainly do it: It gives you full control, and it knows about bleed and where to leave it and where to remove it (where two pages meet at a fold). hth
