Rather than 'I'm wearing uniform', you would say 'I'm in uniform'. For example, we refer to 'a police officer in uniform' to distinguish them from a police officer in plain clothes. Saying that someone is 'In uniform' will be understood to mean that they are wearing uniform.
Your confusion may be because 'uniform' is a noun (with the meaning of a type of clothing outfit) derived from an adjective. As an adjective, it really means uniform outfits of clothing, ie outfits that have 'one form', or a single style. (I'm sure you will know that uni- means one, as in University, unicycle, and unicorn). So its countable when used as a noun, but uncountable when used as an adjective. In your second and fourth examples, its used as an adjective, with the noun (clothing, or outfit) implied.