I can't help with 4b.
4c
Additional information: Although the bacterium Streptococcus pneumoniae most commonly causes systemic bacterial infection in children with sickle cell disease, there are other bacterial species that cause systemic bacterial infection in children with sickle cell disease. Penicillin is effective in treating systemic infections with Streptococcus pneumoniae and is reasonably assumed to be effective in preventing infections when used prophylactically. Similarly, for other bacteria that cause systemic bacterial infection, penicillin is often effective for treatment and might reasonably be assumed to be effective in prophylactic use.