We have measured the flexibility κ of the membranes of a quaternary lamellar phase of periodicity d, without and with particles included between the layers. We show that an increase in cosurfactant amount in the membranes leads to a decrease of the flexibility constant κ, for the two studied systems. On the other hand, the membrane flexibility κ is shown to be hardly different in nondoped and doped (ferrosmectic) systems. This implies that the enhanced value of the elastic bending modulus K, observed when the lamellar phase is doped with particles, cannot be attributed to a simple hardening of the membranes. This leads us to question the well-known relationship between the elasticity of a lamellar phase and the elasticity of its constituting layers: K = κ/d, in the case of ferrosmectic phases.