It's difficult to say exactly since we can't really consider all exploit strategies nor external factors.

But taking the model of "honest" staking at face value, at 49% you can only hope to maintain control for a limited number of blocks. At 50%+, you control the chain forever. That's clearly more than 4% increase.

Again assuming an "honest" staking model you can consider stake as votes and look to voting coalition models such Shapley–Shubik, where staking is viewed as voting between competing chains, and that is trivially superlinear in terms of voting power relative to stake share.

Of course we know that most (all?) PoS systems allow voting on multiple chains, so this may break down. Such systems tend to devolve to PoW though, since stakers are then competing with other stakers to find the combinatorially most-favorable chain state, in which case again control is superlinear in hash power. So I think it is correct.