I wanted to set a field on a MySQL table to one of 4 values for testing purposes. Let's say I want to set the "pet" field to one of {cat,dog,rabbit,hamster}.

First, add a new field to the table:

alter table test add column `id` int(10) unsigned unique key autoincrement;

Now insert each of the four values:

update test set pet = 'cat' where MOD(id, 4) = 1;
update test set pet = 'dog' where MOD(id+3, 4) = 1;
update test set pet = 'rabbit' where MOD(id+2, 4) = 1;
update test set pet = 'hamster' where MOD(id+1, 4) = 1;

Finally, drop the additional field:

alter table test drop column `id`;

I'm always interesting hearing better/alternative ways to do this sort of thing.