Twenty medical scientists have been named as the fourth group of "Scholars in Medical Science" appointed by the John and Mary R. Markle Foundation as part of its program to keep young doctors on medical school teaching and research staffs, John M. Russell, executive director of the fund, announced yesterday. Mr. Russell also announced the decision of the Board of Directors to increase the amount of the grant $1,000 a year, making the 5 year total $30,000 instead of $25,000.