Whilst not in Step record mode …
.
Press(hold) Fill(Page)
Press(hold) Yes
Release Fill
Release Yes
.
Now Fill is latched 
- press Fill again to unlatch
.
This is poorly described in the comments of the Elektron video just released
Edit : Mk2 instructions below
I think it’s more an issue with how easy it is to mix it up with the other latch mode, which is only latched for the duration of one track cycle.
Press Yes + Page to latch Fill for one cycle of the current track.
Hold Page, Hold Yes and then release Page before Yes to latch until Page is pressed again.
so finally 2 patterns in one (so to say)? 
Sugoi !!!
Just started toying around with this. Seems that when you latch FILL on with the YES + PAGE, it only lasts for one cycle. Is that correct?
There’s a fill mode and a latching mode - to latch follow first post exactly
Aha! I see. Ok. I misread the two posts together as the 2nd one being a correction to the first one.
Thanks!
Anybody knows how to do this on mkII?
Yeah - first post is for Mk1 - so I guess you cannot just substitute the dedicated Fill button for page ! If not it might be that this could be sorted to ensure both are equally capable, maybe search the mk2 manual for fill/latch/hold/etc but lots of functions are undocumented
Can’t find anything in the manual.
Me neither, MKI user, but curious.
Whole FILL MODE chapter
10.12.4 FILL MODE FILL mode can be used to create a temporary variation, such as a drum fill, in your pattern. Press [FUNC] + [FILL] to cue FILL mode for one pattern cycle. It will become active when the pattern loops, and remain active until it loops again. You can also activate FILL mode at any time, and for any duration, by pressing and holding the [FILL] key when the pattern is playing (GRID RECORDING mode cannot be active). The FILL mode is active for as long as the key is held.
I wrote support and got this answer:
To latch the FILL mode on the Analog Four MK2 hold FUNC and double-tap the FILL button. Press the FILL button again to unlatch it.
Support poked me about this and I have now updated the Rytm/Four MKI/MKII manuals to cover the latch FILL mode procedures. Hopefully the updated manuals will be out next week.
Fills is something I haven’t used a ton but I’ve started to use more often now…
one thing I’m not sure about is different track lengths/scales.
If I have one track 64 steps and the last page is fill inverted for silence,
and if I have a track set to 1/4x scale it you press yes+page to cue a fill (DT)
It seemed to do the fill on the 64 step track twice…
What takes priority when using different scale per track?