We do not want to generate a line for databases that are already in simple recovery mode.  Add an 'OR' statement if you use bulk_logged
select 'alter database ['+name+'] set recovery simple' from master.sys.databases where database_id > 4 and state_desc = 'online' AND recovery_model_desc = 'full;'

I use your script all the time and have to add that AND recovery_model_desc = 'Full;' part so I thought others might like the addition.