Five planets are known to orbit the star 55 Cancri. The recently discovered planet f at 0.78 AU (Fischer et al.) is located at the inner edge of a previously identified stable zone that separates the three close-in planets from planet d at 5.9 AU. Here we map the stability of the orbital space between planets f and d using a suite of n-body integrations that include an additional, yet-to-be-discovered planet g with a radial velocity amplitude of 5 ms-1 (planet mass = 0.5-1.2 Saturn masses). We find a large stable zone extending from 0.9 to 3.8 AU at eccentricities below 0.4. For each system we quantify the probability of detecting planets b-f on their current orbits given perturbations from hypothetical planet g, in order to further constrain the mass and orbit of an additional planet. We find that large perturbations are associated with specific mean motion resonances (MMRs) with planets f and d. We show that two MMRs, 3f:1g (the 1:3 MMR between planets g and f) and 4g:1d cannot contain a planet g. The 2f:1g MMR is unlikely to contain a planet more massive than ~20 M⊕. The 3g:1d and 5g:2d MMRs could contain a resonant planet but the resonant location is strongly confined. The 3f:2g, 2g:1d, and 3g:2d MMRs exert a stabilizing influence and could contain a resonant planet. Furthermore, we show that the stable zone may in fact contain 2-3 additional planets, if they are ~50 M⊕ each. Finally, we show that any planets exterior to planet d must reside beyond 10 AU.