As you can see in the help centre (which can be found by clicking the ? icon on the top black bar, to the right), upvotes give you +10 reputation, and downvotes cost -2 reputation. Indeed, 4 upvotes and 2 downvotes gives you $4 \times 10 - 2 \times 2 =36$ reputation in total, and since each user can only cast one upvote or downvote per question, $6$ users voted on your post in total.
Regarding your second question, it's not the moderators' job to investigate every single downvote. The script checks once every 24 hours for anomalous voting patterns, and if you have gotten a few suspicious votes, they will be reversed automatically (see this question for more information).
For all the downvotes that were made in good faith, you will have to accept that you may never know the true reason, as Stack Exchange sites do not require downvoters to state a reason at all. This discussion is certainly not new on this site (see this meta post), but if every downvoter was forced to leave a comment, there would always be the danger that the asker/answerer would retaliate. This behaviour is not conducive for the site and I believe that we should try to provide a safe and healthy environment for asking and answering questions on this site, as much as possible. You might want to leave a comment under your post to request that the downvoters explain, but this does not guarantee a response from them.