Decoding an MD5 hash where I already know most of the string Let's say my friend has made an md5 hash of an email address and sent it to me. The email address will be something like 'jessica_XXXX@gmail.com', and I know it begins with 'jessica_' and ends with '@gmail.com'

Does software exist that would allow me to bruteforce the middle part, which is only four unknown characters?
Yes, hashcat is great for this.