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You could have a look at Semillas La Palma if you're not familiar with them yet.
They have a lot of varieties, and quality of the seeds is really good.For "very hot" but not "super hot" some Habaneros are indeed suitable, maybe try "Surinam Red" or "Habanero Peach".
I also like the "Rocoto" variants.. they have a different kind of heat / taste - those usually benefit from really big pots / being grown outside though.
Or Jalapenos?4 Liter is not much in general; if you're planning on starting early (with artificial lighting the first few weeks) you could produce really big plants (if you wanted), given you provide them with big pots, nice soil etc.
Thinking ahead to next year - I want to try a more exotic species. All of the interesting seeds were sold out in Jan/Feb this year, so want to order soon.
This year was lemon drop, demon red and serrano - thinking something in the 'Very hot' range and not 'extremely hot'; https://www.southdevonchillifarm.co.uk/online-shop/chilli-seeds/?features_hash=V5 e.g. a Habanero or Scotch Bonnet?
My biggest pots are 4 litre, 215mm deep/ 180mm wide, so deep-ish and narrow. I could get 1 larger pot for a specific plant - the lemon drops didn't really reach their potential height in these but did get a decent crop (for me).