Limit simultaneous downloads to prevent locking up the machine

I love 4K Downloader as it really simply does, what you'd expect. Sadly, it seems to download all (or at least too much) videos in a youtube playlist at once, which locks up my 2Ghz Core 2 Duo. This makes my machine unusable while 4K Downloader does its black magic. Limiting the number of simultaneous downloads to a user specified number would make this tool even more awesome. Thanks for your time!