Usually when we make a pre-emptive opening bid, we're quite prepared for a negative score, but we're prepared to accept this as the price of making life difficult for opponents to find their optimum [or any] contract.
In 4th position the situation's rather different. We can achieve a non-negative score simply by passing, so our objectives are different now. How different? Well it would be a dumb move to bid with the expectation of going negative. So what we're saying is that we expect to make our 2♥/♠ contract with a scrap of support from partner. Which is likely given that both opponents have passed. So what kind of hand do we need to make this a sensible opening bid?
Firstly we should have a good suit, i.e. including two of the top three honours. Secondly we need to be making our eight tricks opposite a hand that's not particularly suitable, so our range should be about 10-14 HCP. Our objective is a mixture of the constructive and the obstructive: there will be occasions when partner's passed with something close to an opening bid, and will be able to bid or invite game in the knowledge that there's a good 6-card suit opposite. There will be other occasions where a 1-level opening would make it too easy for opponents to get into the action. We're expecting that if partner has a reasonable share of the missing values, we will make our eight tricks, but opponents might have similar expectations in their own suit. So we want to raise the bar before things can get going.
As likely as not, responder will simply pass, but there are other possibilities: