Imperative Socionics (BestSocionics) is a school of thought for Psychosophy (that seems to be independent from the original PY, since some of their articles on their website seem to directly contradict Afanasyev’s work, saying that the position of your Volition doesn't affect your functions, for example).

They created a theory called “Accentuations” (named as "subtypes" on online PY forums for conveniences).

Link to the original theory (with descriptions in the end).
Link to a translated version from Andre Kuney.

Wrote it based on the articles:

According to Imperative Socionics, each function has 3 characteristics:

1) Result-oriented & Unconscious (1 & 4) / Process-oriented & Conscious (2 & 3)
Called Result (1 & 4) & Process (2 & 3) respectively in the translated document.

2) Strong & Subjective (1 & 2) / Weak & Objective (3 & 4)
Called Strong (1 & 2) & Weak (3 & 4) respectively in the translated document.

3) Principled & Introverted (1 & 3) / Unprincipled & Extraverted (2 & 4)
Called Aggressive (1 & 3) & Passive (2 & 4) respectively in the translated document.

An accentuation means that your function can manifest itself differently based on the strength of one of these three main characteristics.

Let’s take 4F - it has Weak, Result, and Passive characteristics. Weak 4F accentuation means that 4F has a strengthened "Weak" characteristic that it shares with 3F (characteristics of 3F - Weak, Process, Aggressive), hence this 4F looks more like 3F, and called in some places as "4F-3" (4F-3F).

So, if your function is focused on some specific characteristic more - let’s say 1L that is focused on results more than anything else - it might superficially take an appearance of a function that shares the same characteristics with it - in this case 4L (shared Result attitude), that makes it a "Result 1L" / “1L-4” subtype.

The subtypes by BestSocionics don't use numbers like this ("4F-3", "3V-1", etc.) and only say what characteristic is accentuated ("Weak", "Result", etc.). The system with numbers ("4F-3") was implemented by people on online PY forums for conveniences. You can use numbers if you want, or just write what characteristic is accentuated, ex.: "Weak 3F", "Result 1V", etc.

It's not stated whether your characteristics have to be accentuated, so if a person doesn’t have a strong predominance of one characteristic over another, it means that this person has no accentuation formed. Whether self-typing as "4F-4" (not having a predominance of any characteristic, 4F not looking like any other F - 1F/2F/3F, and working the way it’s “supposed” to work, paying attention to all characteristics equally) makes sense to your or not is at your own discretion.