In order to study the influence of calcium substitution on the thermal stability and phase composition of the YBa2Cu4O8 (Y-124) superconductor, precursor powders with composition Y:Ca:Ba:Cu = 1 - x : x : 2 : 4 (x = 0.0-0.1) are prepared using an acetate-tartrate based sol-gel method, The phase composition of the products formed after different stages of heat treatment at 800 degrees C and under 1 atm oxygen pressure is examined by X-ray powder diffraction (XRD) and thermogravimetric analysis (TGA). We found that in the first stage of the heat treatment, among stable mixed oxides and carbonate, mainly Y2Ba4Cu7Oy (Y-247) and CuO are formed. Upon further heating Y-247 can react with CuO to form Y-124, but can also decompose into Y-124 and YBa2Cu3O7 - y (Y-123). The formation of Y-123 is favoured by an increasing amount of calcium substitution. We also demonstrate that the TGA technique makes it possible to detect and distinguish between the presence of the Y-123, Y-124 and Y-247 when they are simultaneously present in a sample, while this is less obvious using only XRD.