Вынести подзапросы по click в один подзапрос с группировкой (аналогично как по event)
и сджойнить два подзапроса по дате, как-то так:
SELECT
e.*, c.`click`, c.`unique_click`
FROM (
SELECT
DATE(FROM_UNIXTIME(`inserted_at`)) as `day`,
SUM(IF(`type` = 2, `total_cost`, 0)) as `f_sum`,
SUM(IF(`type` = 3, `total_cost`, 0)) as `d_sum`,
SUM(IF(`type` = 4, `total_cost`, 0)) as `w_sum`,
FROM `event`
GROUP BY `day`
) e
INNER JOIN (
SELECT
DATE(FROM_UNIXTIME(`inserted_at`)) as `day`
SUM(IF(`type` = 1, 1, 0)) as `click`,
SUM(IF(`type` = 2, 1, 0)) as `unique_click`,
FROM `click`
GROUP BY `day`
) c ON e.`day` = c.`day`
ORDER BY e.`day` DESC
LIMIT 10
;