I can't speak for ancient Rome, but medieval Latin authors don't follow a consistent pattern of using IV for four, IX for nine, etc. Most of what I see is iiii or iiij for four (the latter being a later medieval paleographical/typographical quirk to distinguish the final). Like here's a randomly-chosen text printed in 1524 that prints the year as MDxxiiij.
I wouldn't go so far as to claim that as a universal pattern, but it's widely prevalent. I admit that by 1524, Arabic numerals had begun infiltrating western Europe in the economic realm, so this is not perfect evidence, but it does illustrate a more general pattern. Here's a manuscript copied at the end of the 12th century whose table of contents (a new development in book layout, by the way) uses a similar pattern. (Including sometimes running out of space to write the whole numeral! But instead of taking the iv or ix shortcut, it adds in a second line of numeral.)
And that's to say nothing of the medieval authors who "counted" with letters of the alphabet...and then looped back to A when they ran out of letters...
Tag /u/Coniuratos who was also interested and has a great screenname
I assume you're asking because that's how they're denoted in Roman numerals? No, numbers in Latin have their own words, at least until you get into the teens, after which they start being combinations of 1-10 and the closest round number, same as they are in English. In fact, many of our names for numbers in English (and other languages, particularly the Romance languages, as you'd expect) are derived from their Latin equivalents.
For example:
...and so on.
They had words for all their numbers just like we do today. Now, you differentiate between different kinds of number words: cardinal numbers (one, two, three, four...), ordinal numbers (first, second, third, fourth,...), iterative numbers (once, twice, three times...) and number nouns (in english the same as cardinals, used for when you are talking about the numbers themselves, "'one' is a number"). The romans further knew distributive numbers (one of..., a couple of..., a few ...). You can find a list of them all pretty easily online, so i won't make a list of my own and instead link you here and here