4 could be IIII or IV and 9 could be VIIII or IX for example and both are still used today on clock faces for example.
(Mixed Subtractive and Additive notation) On Gate 44 of the Colosseum it is written as XLIIII
Notice that I didn't put a "full stop" after the last sentence? That is because beside using the decimal system for integers the Romans also used the duodecimal system for fractions.
Duodecimal is the division or multiples of 12 and were represented by "." dots and "s" the letter.
"s" was "half or 6/12" and dots & s were used additionally only .....s..... after the Roman Numerals. Like so
. .. ... .... ..... s s. s.. s... s.... s.....So 9 and a half could be IXs or VIIIIs and four and a quarter could be IIII.... or IV.... or IIII:: or IV::
The Colon as we use it today, was believed to be from the duodecimal 2/12 or 1/6, as they often stacked the dots as well as writing them horizontally, like so
.. or
: and so 4/12 or 1/3 was
.... or
: : There was also a usage for Military Legions who would represent the Legion Count of, e.g. the 22nd Legion as IIXX = 2 and 20th.
A stonecutter mistook this as 18 and corrected it to XVIII, perhaps because it is thought that Roman Numerals came from the ancient Etruscan language who were the most advanced population in the early formation of the Roman Empire and wrote 17, 18, and 19 as IIIXX, IIXX & IXX, but then the Etruscan language was written from right to left and so would have been written as XXIII (17), XXII (18) & XXI (19), which could be later interpreted as 23, 22 & 21.
The earlier notations of 500 & 1000 were IↃ and CIↃ, which are believed to have influenced the later D and M.
I believe Ↄ was known as Apostrophus.
The number zero did not originally have its own Roman numeral, but from 525 AD, scholars were known to use the word "nulla" (Latin for "none" ) and from 725 AD the scholar Bede used the notation "N" for "nulla" or for "nihil" (Latin for "nothing" ).
Just by reading these few paragraphs about Roman Numerals, we can see how the invented trading language of Latin has influenced and remained part of modern languages, especially English.
The duodecimal system also remained in the British monetary, weights & measures systems until I was in school.
It makes no difference to my generation when dealing with weights & measures whether decimal or duodecimal, because British people of my age group had to learn both systems, but my parents generation struggle with decimal (metric) weights & measures and likewise, my generation struggle with the duodecimal monetary system, because we were young when it changed to decimal.
The imperial systems were the sole use for all Monetary, Weights & Measures in the British Isles for well over 1000 years and Latin & Frank notation was used in the Monetary system "L S D" until 1971. This is not Roman Numerals, nor is it solely from the Romans, but adapted from the Roman systems and it's notation was spread across Western Europe by Charlemagne (King of the Franks).
L meant Pounds (English) and was from Librae , S was Shillings (English) from Latin Solidi (1/20 of a pound) and D was Pence (English) from Denarii which was from the Roman Silver coins called Denarius (1/12 of a Shilling).
Many monetary denominations were formed from the Librae system, hence the Lira which preceded the Euro in a few European & Mediterranean countries.
The system was easy for those using it all the time, but for me as a child, I had to learn that there were 240 Pennies in a Pound, because there were 20 Shillings in a Pound and 12 Pennies in a Shilling.
There seemed to be no rhyme or reason for the fractions of 20 and 12, so understanding the decimal & duodecimal system mixed, didn't really come easy. It would have made a whole lot of sense if it had been 144 Pennies to the Pound, but to decimalise Shillings and Double it and then Duodecimalise the Pennies, bent the brain of a young boy just starting school.
We also had a lot of coins for various reasons.
Farthing (1/4 Penny)
Halfpenny / Hapenny (1/2 Penny)
Penny (1 Penny)
Threepence / Thruppence / Thrupenny Bit (3 Pennies)
Sixpence (6 Pennies)
Shilling / Bob (12 Pennies / 1/20th Pound)
Florin / Two Bob (24 Pennies / 1/10th Pound)
Half Crown / 2 Sillings & 6 Pence (30 Pennies / 1/8th Pound)
Crown / 5 Shillings (60 Pennies / 1/4 Pound)
10 Shilling Note / 10 Bob (120 Pennies / 1/2 Pound)
Pound (240 Pennies / 20 Shillings)
Guinea (252 Pennies / 1 Pound & 1 Shilling), known as the Gentleman's Pound, mainly used in the purchase of prestigious items.
You should try to get your head around that lot up to the age of 5 and for a few transitional years after.