Something we've done in the past that is pretty easy and low cost, is to use sail repair tape. It is strong / sticky and we never had any issues with numbers coming off. Use stencils to trace out the numbers and then just cut it out - a Xacto knife works great.
A 15 ft roll is $20. You should be able to get around 40 four inch numbers out of that.
Sail Repair Tape
http://www.westmarine.com/buy/bainbr...02_068_002_503
Of course we weren't doing 640!!! numbers!
