If you're using PROC SQL code (and that means if you're writing an expression in the query builder too!!), just a warning that SAS won't do automatic conversion of numeric to character. You'd want a PUT function for that. There's a nice example here (PUT function is about 2/3 down the page):
http://support.sas.com/kb/35/315.html
Once a PUT function does the conversion work in the expression, you could use "||" as you suggest to conbine the strings. You could also use a function like CATS. Something like...
CATS(t1.prefix,PUT(t1.number,4.))