Order is key to labelling pipe. In this case I'd start from scratch (sometimes easier than trying to figure out the problem), delete the line label and the reducers and do the following...
1) Label the line (make sure Fixed Label Format is set to True) and fill out the properties window with the line being 4". Click OK and place the label
2) Insert the two reducer symbols accordingly
3) Right-mouse click on first reducer, select properties, and then select the START NODE. Fill out the details (downstream size 8") and place the reducer label accordingly. This should change the main pipe label to 8"
4) Right-mouse click on second reducer, select properties, and then select the LINE LABEL. Fill out the details (downstream size 4") and place the reducer label accordingly.