Just as a language has syntax and vocabulary, it also has a phonology in the sense of a sound system, phonology describes the way sounds function within a given language or across languages to encode meaning. An important part of traditional forms of phonology has been studying which sounds can be grouped into distinctive units within a language; these units are known as phonemes. Source: Wikipedia Honestly, I read this and I cannot even fully understand what's being said, it's kind of confusing actually, but I guess those four things that you asked are the 'not-so-obvious' characteristics that differenciate one language from another, like the hidden characteristics or properties that many of us who are learning a new language are really not very much aware of.

Phonology studies the way sounds function in a specific language. This means to determine which are the phonemes of a specific language. Phonological system are the group of phonemes of a given language. Phoneme is not the same as a sound. A phoneme is a contrastive unit in the sound system of a language. Contrastive unit means a unit of sound that can distinguish meaning between two words when you have what linguistics call a minimal pair (a pair of words in a particular language, which differ in only one phonological element). for example, in english you have the words "sheep" and "ship" so you can say that /i:/ and /i/ are different phonemes in english because these two different vowels are causing the distinction in the meaning of "ship" and "sheep". you have also the words "pin" and "spin", these two words have two different "p", "pin" has an aspired p ([pʰ]), "spin" has a not aspired p [p]. But in english the sounds [pʰ] and [p] are not phonemes, they are allophones of the same phoneme because you never find a "minimal pair" with these two sounds in english ([pʰ] and [p] never distinguish alone the meaning of two words) . In other languages, like chinese [pʰ] and [p] are two diverent phonemes because you have words like "ban" and "pan" (in chinese pinyin "b" represent a /p/ not aspired and "p" represent an aspired /p/ [pʰ]).

The system of the speech sounds of a particular language.

1. The phonological system can be defined as the set of sound distinctions ("phonemic" oppositions) that native speakers use to express differences of meaning. Many sound differences are perceptible enough to non-natives, but native speakers count them as irrelevant variation induced by the speaker's voice or factors that may affect it (i.e., as merely "phonetic" differences) and do not use them to distinguish meanings. (The difference between "ch" and "q", for example, is phonemic in Chinese, but not in English or Spanish). [Edu v v has already informed you about the "phoneme" idea]. 2. The phonotactic rules determine which combinations of phonemes yield possible syllables in a language. In Chinese, for example, /l/ /p'/, /p/, /t'/, /t/, /k'/....cannot be syllable final, whereas /ng/, /n/, /r/ can. Conversely, /ng/ cannot initiate a syllable, whereas most consonants can, and syllable-initial groups like /tr/, /str/, etc., possible in English, are impossible in Chinese. 3. Tone melodies (intonation) play an important role in all languages, and if you are a trained musician or phonetician you can actually determine the musical intervals between syllables and the amplitude of the oscillations. Consider, for an easy case, how you would pronounce "What" and "he" in "What did he say he wanted?" In tonal languages like Chinese, furthermore, intrasyllabic tone oscillations are phonemic, since they determine differences of meaning (e.g., shi1/shi4/shi2). 4. Stress also plays a role in all languages (look at how you pronounce "in the SHOPS near the uniVERsity = pa-pa-PÁ-pa-pa-pa-pa-PÁ-pa-pa), although not all languages use stress alone to express differences of meaning. One that does, though, is Spanish: "I sing" is /KÁN.to/, whereas "He sang" is /kan.TÓ/. Such a neat distinction between stress and tone, however, is artificial. As a matter of fact, they both are, basically, articulatory energy and overlap to a great extent, but they are conceptually different and I suppose you do not expect here more than a very elementary characterization of the concepts you ask about.

The sound system of saying a word in any language.