Assalamu 3alykum Munir Rana
I hope you dont mind me giving some input on this.
Since we know there is a difference between messengers and prophets, the Quran defines and affirms clearly that prophets are given scriptures and authority. It also defines and affirms that messengers reaffirm the Scriptures that is between their hands and/or to warm of the last Day.
Its also important to note that
prophets are automatically by default messengers since 2:213 confirms they also give good news and warnings. So keeping this in mind that prophets are by default and are automatically messengers, then its worthy to note if the messenger that is being mentioned is also a prophet? If he is, then there is absolutely nothing wrong with referring to that person as a messenger with which the scripture is sent down to him as it does hold true.
If the messenger or messengers that is being mentioned in a specific verse is also a prophet then that messenger would also have a scripture sent down with him. The reference in verse 4:136 is to believe in the messenger and what was sent to him and not to define who the messenger is per se.
So in this instance when God is asking the people to believe in His messenger and the scripture which He has send to him we know that this messenger is also a prophet, and its expected that the people understand and know that Muhammad(pbuh) is also a prophet and the seal of the prophet. So in this verse the instruction to believers is to believe in the messenger and the scripture that was sent to him.
There is also a related post
[1] that brother Joseph responds to that I share below.
Inshallah this helps.
Salam
[1] 57.25
http://quransmessage.com/forum/index.php?topic=294.msg923#msg923