Internal combustion engine system, comprising: - an internal combustion engine comprising a cylinder block housing a plurality of cylinders, a first intake manifold connected to a first group of cylinders, a second distinct intake manifold connected to a second group of cylinders and an exhaust manifold (8, 8a) for receiving the exhaust gas emitted from at least the first group of cylinders; - an air inlet line; - an EGR line connected to the exhaust manifold (8, 8a); wherein the internal combustion engine system is operable in at least two operating modes, respectively a normal operating mode in which all cylinders are supplied with fuel and a cylinder deactivation mode, in which the cylinders of the first group of cylinders are no longer supplied with fuel nor fresh air, characterized in that: - the system also includes a mixing unit comprising a four-way valve, said four-way valve having a first inlet connected to the EGR line, a second inlet connected to the air inlet line, a first outlet connected to the first intake manifold and a second outlet connected to the second intake manifold; - the four-way valve is designed so that, in said normal operating mode, the intake gases supplied to the first intake manifold and to the second intake manifold have approximately the same proportion of exhaust gas and fresh air and so that, in said cylinder deactivation mode, the intake gas supplied to the first intake manifold only includes exhaust gas and the fresh air is directed exclusively to the second intake manifold. - an internal combustion engine comprising a cylinder block housing a plurality of cylinders, a first intake manifold connected to a first group of cylinders, a second distinct intake manifold connected to a second group of cylinders and an exhaust manifold (8, 8a) for receiving the exhaust gas emitted from at least the first group of cylinders; - an air inlet line; - an EGR line connected to the exhaust manifold (8, 8a); wherein the internal combustion engine system is operable in at least two operating modes, respectively a normal operating mode in which all cylinders are supplied with fuel and a cylinder deactivation mode, in which the cylinders of the first group of cylinders are no longer supplied with fuel nor fresh air, characterized in that: - the system also includes a mixing unit comprising a four-way valve, said four-way valve having a first inlet connected to the EGR line, a second inlet connected to the air inlet line, a first outlet connected to the first intake manifold and a second outlet connected to the second intake manifold; - the four-way valve is designed so that, in said normal operating mode, the intake gases supplied to the first intake manifold and to the second intake manifold have approximately the same proportion of exhaust gas and fresh air and so that, in said cylinder deactivation mode, the intake gas supplied to the first intake manifold only includes exhaust gas and the fresh air is directed exclusively to the second intake manifold. - the system also includes a mixing unit comprising a four-way valve, said four-way valve having a first inlet connected to the EGR line, a second inlet connected to the air inlet line, a first outlet connected to the first intake manifold and a second outlet connected to the second intake manifold; - the four-way valve is designed so that, in said normal operating mode, the intake gases supplied to the first intake manifold and to the second intake manifold have approximately the same proportion of exhaust gas and fresh air and so that, in said cylinder deactivation mode, the intake gas supplied to the first intake manifold only includes exhaust gas and the fresh air is directed exclusively to the second intake manifold.