"Challenge 4a" 用英语表达为 "Challenge 4a"，没有翻译的必要。这个词组来源于具体上下文。例如:
"We have to complete Challenge 4a before moving on to the next level."
(我们必须在进入下一关之前完成挑战4a)
"Challenge 4a is the most difficult one in this game."
(挑战4a是这个游戏中最难的一个)
"I can't bepeve I finally passed Challenge 4a!"
(我真的不敢相信我终于通过了挑战4a!)