You may use multiple IF statements in the same eval, just remember to close them all.
For example:
| eval StartNum = if (
substr(TwitterID,1,1) = "0", 0,
if(substr(TwitterID,1,1) = "1", 1,
if(substr(TwitterID,1,1) = "2", 2,
if(substr(TwitterID,1,1) = "3", 3,
"over 3"
))))
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