"Mi-ar plăcea o vacanță de patru săptămâni."
Translation:I would like a vacation of four weeks.
In this case, you should leave the "s" off of "weeks". "A four week vacation". But, if you arrange it as Rowlena-L did, then the "s" stays. I'm not sure I can explain why, but I am a native English speaker.
I think it is similar to when talking about age. "I have a five year old son." NOT "I have a five years old son."
If you say "I would like four weeks of vacation", that's correct. But if you want to say "I would like a four week vacation," then yes, you leave off the s at the end. It is also (possibly more) grammatically correct to write it as four-week. The difference is that in the first example, "weeks" is a noun, which is plural when you talk about more than one of them, hence the s. In the second example, though, "two week" or "two-week" is an adjective, describing "vacation" which is the noun - and in this case, we don't add an S to denote plurals. As you say, it goes for any age or time description (days, weeks, months, years etc). That's my understanding, anyway, and I stand to be corrected by the numerous people who are far better at grammar than I am!
There are 2 ways of looking at this sentence depending on which is the subject - ”me” or ”the vacation”.
1) I would like a vacation / I would wish for a vacation. In this case you can use "Eu aș” - ”Eu (mi-) aș dori o vacanță” / ”Eu aș aprecia o vacanță”
2) A vacation would please me. In this case you can use ”ea ar” or ”ea mi-ar” (to me) - ”O vacanță mi-ar plăcea” / ”Mi-ar plăcea o vacanță”
The second case is similar to italian (as I see you study italian also): Mi piacerebbe una vacanza. You use ”piacerebbe” which is the 3rd person (la vacanza) and not ”mi piacerei” that is the 1st person