Radio observations in the ammonia inversion lines of four comets, C/2001 A2 (LINEAR), 153P/Ikeya-Zhang, C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR), were performed at the Effelsberg 100-m Radio Telescope during their respective close approaches to Earth. None of the four lowest energy metastable lines (J, K = J), J = 1–4, could be detected in these comets. We derive the following 3σ upper bounds on the NH3 production rate , and comparing to the corresponding water production rates, percentage NH3 abundances relative to H2O: Q(NH3) < 1.9×1026 s−1 (0.63%) for C/2001 A2 (LINEAR), Q(NH3) < 2.7×1026 s−1 (0.13%) for C/2001 Q4 (NEAT), Q(NH3) < 2.3×1027 s−1 (0.74%) for C/2002 T7 (LINEAR) and Q(NH3) ≤ 6.3×1026 s−1 (0.63%) for Comet 153P/Ikeya-Zhang. At 0.74% or less, the ammonia-to-water ratios are factors of ∼2 below the value for C/1995 O1 (Hale-Bopp) and 1P/Halley, suggesting chemical diversity between comets. The 18-cm lines of OH were clearly detected in the two comets observed during the 2004 campaign, thereby validating the cometary ephemerides.