you may first look your fasta reference you created. There might be spaces in the fasta header, which might be included in the alignment (samtools view -H ) but might not be carried over to the bed-file.
Your 4th column is a read name. Meaning, you converted each alignment of the bam-file into bed-format.
Alternatively, you can use genomeCoverageBed to produce a coverage-profile.
ucsc provides definition for format like bed and bigBed definition are available here : https://genome.ucsc.edu/FAQ/FAQformat.html
In your case I don't understand your bed. usually a bed is chromosome start end and optional column. Hope the link I provided may help you.