As response to continuously increasing demands on transmission capacity, we describe a four-dimensional (4D) coded multicarrier/OFDM schemes suitable for metro and long-haul optical networks, which are capable to fully exploit advantages of both multicarrier/OFDM and multidimensional systems. Two multidimensional coded modulation schemes are presented in this invited paper: (i) 4D multicarrier system in which the 4D signal constellation points are imposed on orthogonal subcarriers and (ii) N-dimensional OFDM scheme. In N-dimensional OFDM, the signal constellation points are represented in matrix form by placing coordinates of signal constellation points along columns of signal matrix. We further apply 2D inverse FFT to obtain 2D array of complex numbers. Coordinates of complex numbers can be considered as I and Q channels, while coordinates of 2D array can be mapped to two orthogonal polarizations; resulting in 4D signal space. On receiver side, we use conventional polarization-diversity receiver, followed by 2D-FFT demapper. Therefore, these schemes can fully exploit advantages of OFDM, to deal with chromatic dispersion, PMD and PDL effects; and multidimensional signal constellations to improve dramatically OSNR sensitivity of conventional optical OFDM systems.