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Two levels of non-refractory (V and Ti) and refractory (Mo, Nb and Ta) alloying elements were added to Co-9Al-10W ternary alloy (at.%, baseline alloy) in order to investigate alloying effects on phase equilibria, microstructural features of γ' phase and secondary phases at 1300°C and 900°C, respectively. It is suggested that refractory alloying additions promoted the precipitation of the μ phase at 1300°C. The phase equilibria changed from γ single-phase region in the baseline alloy to γ+μ two-phase region in quaternary alloys containing high levels of refractory alloying elements (4 at.%) at 1300°C. The μ phase was enriched in W, Mo, Nb and Ta, but depleted in Al, V and Ti. For quaternary alloys with low levels of alloying additions (2 at.%), the γ/γ' two-phase microstructure was observed after aging treatment at 900°C for 50h and 300h, but small amounts of the plate-like DO 19 phase precipitated in the matrix of 2Mo and 2Nb alloys. The volume fractions of the γ' phase were at very high level in the range of 74% to 83% in all these experimental alloys. Compared with the baseline alloy, alloying additions did not show the strong effect on γ' morphology, which remained cuboidal after long term aging except Mo addition. For quaternary alloys with high levels of alloying additions after aging at 900°C, the DO 19 phase precipitated in addition to γ and γ' phases, as well as the blocky μ phase inherited from the precipitation at 1300°C. There were two types of the DO 19 phase: one was plate-like and precipitated in the matrix; the other was blocky and transformed from the previous μ phase. Again, refractory alloying additions (Mo, Nb and Ta) promoted phase transformation for both types of the DO 19 phase. The current study is helpful to understand phase equilibria and microstructure in Co-Al-W-base alloys and further develop multi-component γ'-strengthened Co-base superalloys.