For 4a, the longest chain is 7 (as you've highlighted) and there's 3 additional groups. So your main chain is heptane and regardless of where you start numbering on this one, you have 3,5-dimethyl and 4-propyl (C3H7 on the 4th carbon). Now you just put everything together. You write the name in alphabetical order so dimethyl goes first so the name is: 3,5-dimethyl-4-propylheptane.