Chemical composition of synthetic gas and concentration of its components depend on gasification process and feedstock. Synthetic gas mainly consists of hydrogen (H-2) and carbon monoxide (CO), and may contain trace amounts of carbon dioxide (CO2) and methane (CH4). In this study, combustion and emission characteristics of H-2/CO/CH4/CO2 blending syngas mixtures with high H-2/CO ratio were experimentally investigated in a swirl-stabilized premixed combustor. Furthermore, stable operating ranges (flashback and blowout equivalence ratios) of such mixtures and flame response of 67.5%H-2-22.5%CO-5%CO2-5%CH4 mixture to acoustic forcing were also detected. During the experiments, CH4 amount in tested gas mixtures varied between 5% and 20% by volume (at intervals of 5%), and H-2/CO ratio and CO2 concentration were kept constant at the values of 3 and 5%, respectively. As a consequence, mixtures of 67.5%H-2-22.5%CO-5%CO2-5%CH4, 63.75%H-2-21.25%CO-5%CO2-10%CH4, 60%H-2-20%CO-5%CO2-15%CH4 and 56.25%H-2-18.75%CO-05%CO2-20%CH4 were derived and tested under the same boundary and physical conditions. All experiments were conducted at 0.4 equivalence ratio (phi) and 0.2 geometric swirl number (SN). Preliminarily, stable operating ranges of respective gas mixtures were determined to find an equivalence ratio at which all gas mixtures could be tested without causing any flame instability and then, CH4 addition effects on combustion and emission characteristics of such mixtures were evaluated via utilizing axial and radial temperature, CO and O-2 profiles. Lastly, flame behavior of 67.5%H-2-22.5%CO-5%CO2-5%CH4 mixture was evaluated under externally modified acoustic conditions. Acoustic field in the combustor was mechanically altered by using side-mounted loudspeakers in the frequency range of 30-200 Hz. To identify flame instabilities and to evaluate the degree of coupling between heat release and pressure oscillations, pressure sensors and photodiodes installed on combustion chamber and burner were utilized. Results of this study showed that CH4 addition to synthetic gas mixtures improves rich flammability limits, and increases flame temperature, flame height and emissions. Under externally modified acoustic conditions, flame behavior slightly alters (flashback and blowout does not occur). Additionally, pollutant emissions improve in an environment-friendly manner under such conditions.