I suggest you do a search and look at what transcription is paying on average. If you feel you are worth more then the average then by all means charge it. Don't be afraid to charge what you are worth.
Agreed with Suzzane. You have already done 4 successful assignment with good rating, so there is no reason for charging lesser than you are worth. You may charge whatever you are charging at local level. If you compare with your peers on upwork there will be wide range of rate.