I've just began learning some Kanji by looking-up how the numbers are written (and read) and suddenly bumped into confusion while watching some tutorials. I can hear the tutors pronouncing a number in on-yomi (just like in a dictionary, where all on-yomi are written in Katakana) but for some reason they write these number Kanji in Hiragana (instead of Katakana). Is it to make the learning process easier by keeping attention away from yet another script (Katakana) or do they write on-yomi with Katakana only in dictionaries whereas in real life they actually write on-yomi with Hiragana (just like kun-yomi is written with Hiragana in dictionaries)?
they write on-yomi with Katakana only in dictionaries whereas in real life they actually write on-yomi with Hiragana (just like kun-yomi is written with Hiragana in dictionaries)?
↑ This is the answer. Using katakana to indicate on-yomi is one convenient way to distinguish on and kun, but not all dictionaries do so.
In everyday life, the reading of 五, for example, is usually written as ご (hiragana) even if it's on-yomi. Take a look at some random questions in this site and you can find that readings of kanji are almost always indicated in hiragana.
Because it's a long tradition of kanji dictionaries, succeeded from the "first Japanese kanji dictionary" 漢和大字典.
Technically, kanji were foreign notions in Japanese; on'yomi was the pronunciation, and kun'yomi was its definition in Japanese. Therefore, it's very natural that they used katakana to mark the former, and hiragana for the latter. Suppose there's a dictionary entry fiancé — it's the kanji; and next to the headword is [fee-ahn-SEY] — it's the on'yomi; following it will be written "betrothed" — this is the kun'yomi.
For this reason, every dictionary that explains kanji writes on'yomi in katakana and kun'yomi in hiragana to this day, even though we use them both in hiragana unless there are special meanings.
In general, outside of specialised contexts like dictionaries and books for young children, Japanese words are written in hiragana while non-Japanese words are written in katakana or romaji. Since the Japanese numbers are Japanese words, they should normally be written out in hiragana (いち、に、さん、よん). For most practical purposes, of course, they are written in Arabic numerals (1,2,3,4) or kanji (一、二、三、四).
It's also the case that many Japanese courses for foreigners teach hiragana first, katakana second, and kanji later. Since the numbers are always an early topic when learning a new language, it is likely that only hiragana will be known by the student at the time (besides romaji, that is).