Number Meaning

One:
One is often associated with being the best (I am number one)
"The Pythagoreans referred to number one as the "monad". It symbolized the creator. It was a male number. Number one represented strength." (www.mysticalnumbers.com)

Two

"Two is the number of balance, but also the number of conflict and opposition.

Male and Female	Light and Dark	Day and Night
Good and Bad	Life and Death	Sun and Moon

(www.mysticalnumbers.com)

"The symbolic meaning of number Two is kindness, balance, tact, equalization, and duality" (http://www.whats-your-sign.com)

Three
"The spiritual meaning of number Three deals with magic, intuition, fecundity, and advantage. The number Three invokes expression, versatility, and pure joy of creativity. Three is also a time identifier as it represents Past, Present and Future." (http://www.whats-your-sign.com)
"Three was considered the number of harmony, wisdom and understanding" (www.mysticalnumbers.com)


Four
Four is an unlucky number in Chinese, due to it's similar sound to the chinese word for 'death', although this pronounciation can difer with the different dialects.
"Number four is the number of stability, order and completion of justice. Number four is the number of the earth and mankind." (www.mysticalnumbers.com)

Of these, I'm going to go with the number 4, since that is the only number I can find that has 'unlucky' connotations.  Also, since I am making a skull, the 'death' association seems appropriate.

Bibliography

Wikipedia, viewed 20th September <http://en.wikipedia.org/wiki/Numbers_in_Chinese_culture#Unlucky_numbers>
Mystical Numbers, viewed 20th Sept <http://www.mysticalnumbers.com/Number_2.html>
What's your sign, viewed 20th September <http://www.whats-your-sign.com/spiritual-meaning-of-numbers.html>