Is it common to use the word "boast" and "career" together?
"Boast" and "career" are two separate words here and they are not necessarily used together often. To "boast" in this context means to possess something in a prideful way, while the word "career" refers to the albums they've produced throughout their "career" as a band. So the sentence here means that BTS possess four No. 1 albums in the billboard 100 chart throughout their career.