Interference mitigation can be particularly challenging in hybrid 4G cellular networks employing a femtocellular tier, especially given the ad hoc nature in which femtocells are deployed. So far, mitigation mechanisms have been based on simplified models assuming regular (grid-based) deployment, using average interference characterization or satisfying the SINR requirements while ignoring the required data rates. In this paper, we show that such simplifications can be relaxed. Specifically, we derive an interference mitigation mechanism assuming an ad hoc deployment. The mechanism optimizes resource allocations on per-subcarrier basis while satisfying user data rate requirements. It is also based on a rigorous cross-tier interference characterization (macro-femto, femto-femto), and utilizes robust stochastic programming techniques. Simulation results indicate great potential in our proposal as it significantly outperforms a heuristic greedy algorithm.