As a matter of math, if the pre-money valuation is $3.7B, and the investors put in $200M (for 5.1% of the company at a $3.9B post-money valuation), then any subsequent sales (in secondary markets or IPO or as an eventual public company) at any valuation higher than $3.9B would be a gain.

With liquidation preferences, it's possible that even an immediate sale to an acquirer for $3.9B might give these latest investors some guaranteed return.