combining deletes

[yawnmoth]

How would I go about combining the following deletes into a single
delete operation that's compatable with MySQL 4?

DELETE FROM some_random_table WHERE a=1 ORDER BY b ASC LIMIT 2
DELETE FROM some_random_table WHERE a=2 ORDER BY b ASC LIMIT 2
...
DELETE FROM some_random_table WHERE a=MAX(a) ORDER BY b ASC LIMIT 2

Also, the above DELETEs deletes two rows for each a. How could I make it so that all but two rows are deleted for each a? Although a single query would be preferable, multiple queries would be sufficient.