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The [CII] 158 μm far-infrared (FIR) fine-structure line is one of the most important cooling lines of the star-forming interstellar medium (ISM). It is used as a tracer of star formation efficiency in external galaxies and to study feedback effects in parental clouds. High spectral resolution observations have shown complex structures in the line profiles of the [CII] emission. Deep integrations with the SOFIA/upGREAT 7-pixel array receiver in the sources of M43, Horsehead PDR, Monoceros R2, and M17 SW allow for the detection of optically thin [13CII] emission lines, along with the [12CII] emission lines, with a high signal-to-noise ratio (S/N). We first derived the [12CII] optical depth and the [CII] column density from a single component model. However, the complex line profiles observed require a double layer model with an emitting background and an absorbing foreground. A multi-component velocity fit allows us to derive the physical conditions of the [CII] gas: column density and excitation temperature. We found moderate to high [12CII] optical depths in all four sources and self-absorption of [12CII] in Mon R2 and M17 SW with high column densities for the warmer background and colder foreground.