Why does the word for "Wednesday", <thứ tư>, use the Sino-Vietnamese word for "four" when the rest of the days use native Vietnamese numbers?
That is a rather observant question! Firstly, the Vietnamese count their days of the week rather than assigning specific names to them (except for Sunday). However, maybe due to the influence of early European missionaries themselves, the first day of the week is actually Sunday with Monday being the 2nd, Tuesday the 3rd etc. With the exception of Sunday (Chủ nhật = the primary day, or the day of the lord) the rest of the days are given a number from 2 to 7 (Monday to Saturday).
Sunday = Chủ nhật
Monday = Thứ hai
Tuesday = Thứ ba
Wednesday = Thứ tư
Thursday = Thứ năm
Friday = Thứ sáu
Saturday = Thứ bảy
Besides Sunday, the only other odd one out SEEMS to be Wednesday. 'Thứ' can mean a type or sort of something but can also mean 'order/rank'. The word thứ in this context may be a contraction of 'ngày thứ' (day number, day order/rank). Generally speaking, the ordinal numbers in Vietnamese are actually: nhất, nhì, ba, tư, năm with the rest of the numbers being the same as the cardinals. The cardinal numbers from 1-5 are: một, hai, ba, bốn, năm. Apart from the two, there is also a rarely used Sino-Vietnamese (Chinese borrowed) number system mostly relegated nowadays to more academic terms.
Number: 1, 2, 3, 4, 5
Cardinal: một, hai, ba, bốn, năm
Ordinal: nhất, nhì, ba, tư, năm
Sino-Viet: nhất, nhị, tam, tứ, ngũ
Technically, if the days of the week are being counted like ordinal numbers, then why is Monday not thứ nhì but rather thứ hai? That is because the cardinal for the number 2 'hai' can also be used colloquially to mean 2nd. Hence, if you factor in that irregularity, you end up with the rest of the numbers, 3-7 being regular. Wednesday is thứ tư instead of thứ bốn because the days are being counted (ordinal numbers).
As to the Chữ Nôm, native Vietnamese numbers were given new characters as shown in the examples VROOR has given. Every number from 1-9 except for 1 and 4 is unique to Chinese. 1 is simply a borrowing of the character 没 which is actually pronounced một in Sino-Vietnamese as well. Similarly, the ordinal number 4 'tư' borrows the character 四 for its rough pronunciation. Of course, as with the number 2 the ordinal number for 4 is a corruption or nativisation of the Chinese. However, you will also see a lot of Chữ Nôm writers use the original simple Chinese number characters to represent native Vietnamese numbers. ie. 一, 二, 三, 四, 五 for một, hai, ba, bốn, năm. Therefore, there could be confusion with the Sino-Vietnamese numbers nhất, nhị, tam, tứ, ngũ but that is quite infrequent.
Vietnamese also count their months in cardinal terms. Tháng (month) + a cardinal number from one to twelve. So once again, month two (February) is Tháng hai instead of Tháng nhì, April is Tháng tư instead of Tháng bốn.
Chữ Khoa Đẩu ‧字蝌蚪
Chữ Hán ‧字漢
Chữ Nho ‧字儒
Chữ Nôm ‧字喃
Chữ Quốc Ngữ ‧字國語
In Vietnamese, the 字 precedes the style it represents, not after the definition of style.
Actually I wrote 漢字, 喃字, 國語字 etc. using the Chinese terms for them, not the Chữ Nôm.
P.S: I'm quite fluent in Vietnamese because I was raised in a Vietnamese family although I'm ethnically 1/2 Chinese.