Let me ask if I will use PDF-X/4 I will not have got any RGB-> CMYK conversion.

The default PDF/X-4 preset makes no color conversions, but you can convert everything to a CMYK space and still have a PDF/X-4 that will pass X-4 compliance in Acrobat's preflight.

I do not understand why using PDF-X/4 I have got some fonts rasterised - looks badly.

I use Flattening transparent artwork settings high.

You can't flatten transparency and still export a PDF that meets the X-4 standard, so if you can choose a flattener preset in the Advanced tab (it's not grayed out), then you're no longer exporting a PDF/X-4

Even though the preset name hasn't changed this would not export a valid PDF/X-4

PDF/X-4:2008 available. No PDF/X-4:2010

There's little difference, see this thread: http://forums.adobe.com/thread/838266

One thing to note about PDF/X-4 is that all color is profiled. So if you export to PDF/X-4 and convert everything to Fogra27 CMYK, all of the CMYK objects will have the Fogra27 profile. If your printer outputs to a different profile, something like US SWOP, all of your CMYK will get reconverted. If that's the case you'd be better off not not converting to CMYK on export and leaving it to the printer.

If the printer is requires you to deliver all CMYK color then PDF/X-1a is better because no profiles are included—your CMYK values will be output with no further conversions.

PDF/X-1a may in fact include a profile

How would I do that without changing the Standard to None? Isn't there a difference between an Output Intent and a Profile? If I export a PDF/X-1a and place it back into ID there's no profile—at least the placed CMYK values remain unchanged no matter what the document profile is.

There is nothing that prevents a printer with a PDF/X-1a workflow to do a CMYK=>C'M'Y'K' conversion if they believe that the specified printing condition doesn't match their actual printing condition.

I understand that, but there are printers who still request all CMYK usually because they don't want any kind of conversion on their end good or bad—they are not likely to force a conversion. Note that the OP's printer in unresponsive regarding profiles.

If I make a page with a frame filled with a color where it's easy to spot a conversion—something like 0/0/10/50—and export it as PDF/X-1a, AcrobatX's preflight tells me the profile is DeviceCMYK not the Fogra27 Output Intent Profile I exported.

If I open Acrobat's Output Preview, the default Simulation Profile is the output intent profile and the gray shows the same 0/0/10/50 separation as in ID. If I change the Simulation Profile to US Sheetfed the separation panel does not show a conversion—I still get 0/0/10/50.

If I  print separations to the different destination profiles the results match the Output Preview, in both cases the cyan and magenta plates are blank. I don't see anyway to force a CMYK-to-CMYK conversion from the Print dialog.

If I export the page again and include a destination profile rather than an output intent (by setting the Standard to None) I get very different results.

Preflight shows Fogra as the profile. In this case the Output Preview simulation defaults to my Acrobat working CMYK space and I get a conversion (because it is not Fogra). If I select Fogra as the simulation there's no conversion, which is what I expect. Again my printed separations match the simulations—by default I get a CMYK-to-CMYK conversion.