You can use the tool parallel like follows:

ls *.bed | parallel -j 4 'bedtool ... {} > {}.coverage'

The syntax is close to that of 'find'

The tool you need is the venerable 'make' with the option '-j,

   -j [jobs], --jobs[=jobs]

        Specifies the number of jobs (commands) to run simultaneously.

here is a Makefile as an example. the coverage files can be generated in parallel, but merge.txt will have to wait for them:

#pattern rule: how to make a *.coverage from a *.bed
%.coverage:%.bed
    echo "do something with $< and generate $@" > $@

#your list of *.bed
bed.files=file1.bed file2.bed file3.bed file4.bed

#special targets that don't really exist
.PHONY: all clean

#top target 'all' needs merge.txt
all: merge.txt

# target 'merge.txt' needs the coverage files 
merge.txt: $(patsubst %.bed,%.coverage,$(bed.files))
    #loop over the bed file and create merge.txt
    echo "creating $@ from $(foreach BED,$^, bed file ${BED})" > $@

#cleanup
clean:
    rm -f merge.txt $(patsubst %.bed,%.coverage,$(bed.files))

invoking make -j 4 should produce the following output:

$ make -j 4 

echo "do something with file1.bed and generate file1.coverage" > file1.coverage
echo "do something with file2.bed and generate file2.coverage" > file2.coverage
echo "do something with file3.bed and generate file3.coverage" > file3.coverage
echo "do something with file4.bed and generate file4.coverage" > file4.coverage
#loop over the bed file and create merge.txt
echo "creating merge.txt from  bed file file1.coverage  bed file file2.coverage  bed file file3.coverage  bed file file4.coverage" > merge.txt

As a general approach with a job scheduler, say Sun Grid Engine, there is qsub with the -hold_jid option to submit a task that waits for the successful completion of parent qsub-ed grid jobs.