Depends who you are prtecting it from. A 4 digit number gives a possible 10,000 different combinations so would be quite safe from the average user.
If someone is determined, they may 'research' you (birthdate, family birthdates etc) and try those numbers first but again depends on how likely this is to happen given your particular situation.
Worth pointing out that no excel document is completely secure - any doc can be opened regardless of protection if the correct process, macros and software are used. Always worth laying in additional protection (place the doc in a secured area of your HD for example).