Your 1st Question
bro, can i know whats the category of very good kda ? is it around above 5 kda?
Bro 4 KDA can be a very good KDA, you must remember its not only KDA that matters, almost all the stats are counted while calculating your MMR. So, till the time you are in VHS you need not to worry, but if you get even 1 solo match in High Skill or Normal skill then thats alarming, because that means your MMR is going critically down and you are definitely below 4k.
Now 2nd Question
is that has any bad effect to my mmr if i keep partying with him?
No bro, it does not effect your Solo MMR, its only your Party MMR that is effected once you lose or perform bad in a party match. Solo MMR is only updated once you play in solo queue.