Cowboy says that the Thule Yepp Nexxt Maxi is compatible with the C3, C4, and C4 ST. Thule's Yepp Nexxt Mini is not compatible with the latest Cowboy 4 and 4 ST models, but it will work with the third-generation Cowboy 3. As far as child car seats go, Cowboy's top recommendation is the Hamax Caress Child Seat which it sells through its online store (simply add it to your order when purchasing a bike).