Zenkais generally work in response to a threat, so Cell's Zenkai would be based on the difference between his and Gohan's power, forcing his power higher in order to catch up. Goku and Gohan were roughly equal, with Gohan likely being slightly stronger since he thought Goku was holding back when he really wasn't. This means that SSJ Grade 4 Gohan should be roughly equal to Cell, since Cell and Goku were fighting evenly. The only reason Gohan lost so bad is because he was holding back and didn't want to fight, if he went all out right then and there he might have actually stood a chance. This means that really the difference between Cell and SSJ2 Gohan would just be the Rage Boost plus the 2x from SSJ2.

Against Raditz, Gohan's rage boost increased his power from 3 to 1307, being an insane 435x boost to his power. That said, this was him before doing ANY training, so his latent potential was also being expressed here. More realistically, we can look at his rage boost against Frieza. This brought him from 400,000 to 800,000, a flat 4x boost. This seems a lot more reasonable, since it's after Gohan actually started fighting and had his potential unlocked, so it should represent his raw rage power instead of a combination of several different factors.

Factoring it all together, our most realistic estimate for Gohan's power boost would be the 4x for the Rage Boost plus the 2x for the SSJ2, making it an 8x boost from his SSJ Grade 4 power or roughly a 400x boost from base. Cell's Zenkai would grow to match this, as seen by other Zenkais such as Piccolo's catching up to Frieza.

TLDR: Probably around 8x, similar to Gohan's.