Only you vulnerable, you have a 13- high card point hand like the following: (s)x (h) AJxx (d) KJxx (c) KJxx. (No x is higher than a 7). Three passes have preceded you.
The "Rule of 15" suggests that you open in fourth seat only if the sum of your high card points and spades add up to at least 15. It works most often for 11 point hands with four spades. Here, you have the full opening point count (13), but are weak in spades, the highest ranking suit.
On "average," your partner will have nine, or one third of the remaining 27 points, giving your side about 22 (out of forty). Balanced against this is the near-certainty that the opponents own the spade suit. (Partner would have opened with a "weak two" spades with six, unless they were very weak.)
Does this consideration suggest that you should pass in fourth seat, even though you might have opened, say one diamond, in first, second, or third seat?
Edit: I am now also assuming unfavorable vulnerability, that is "only you vulnerable" (the original version of the question said both vulnerable). Does vulnerability make a difference in this borderline case?