公司不按全职支付福利 and provide T4A-slip has been wrong first, But T4A-slip do allow you todo your deduction temporary. You can't 按全职交税 without T4-slip. You can file a claim by assist of Ministry of Labour to get back your money and benefits from your former company in 6 months to 2 years.
If Revenue Canada considers you are not a contractor, You can file a claim by assist of Ministry of Labour to get back your money from your former employer in 6 months to 2 years.
Ministry of Labour Offices Locationand Telephone Number:
"Is it important for me to file a claim within a certain time?"
Here is reference for two years to file a claim -- http://www.gov.on.ca/LAB/esa/esa_e/claim_e.htm
Page Link: http://www.gov.on.ca/LAB/esa/esa_e/claim_e.htm