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Tomato fertiliser works well for chillies. This was the last big harvest, don’t know all the varieties but they were mostly a bit underwhelming. The split yellow fellows at the bottom were excellent, I shall try to overwinter the plant (apparently prune to ~20cm of stem, then v scant watering) and save some seeds. Paper Lanterns were good also & I’ll do the same; smallish plant too which is convenient.
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.