According to a study of cases at the APCC of homemade playdoh ingestion, a dose of salt >1.9 g/kg can be toxic and >4 g/kg can be lethal. 15 ml of salt weighs about 18 g. So if the dog ate about 75 ml of salt, the dose would be 3 g/kg so signs of hypernatremia (ataxia, tremors, seizures) can be seen.
1 Tbs. = 15 ml = 18g
4 Tbs would therefore be 72g of salt
If the dog is 40lbs (72g divided by 40lbs/19kg aprox.) this would mean he ingested 3.78g/kg.