Will I be automatically promoted to E5 when I accept the new position?

No, you will not automatically be promoted. You are simply an E4 in a E5 position doing the work of an E5.

No, you will wait.
You are promotable on an MDAY list. You were not on the AGR promotion list this year.
You will be on the next AGR promotion list, and will promote off that.

Most entry level National Guard AGR positions are at the E-5 level. Generally speaking once your begin the position and are MOSQ you will be promoted assuming you have completed BLC and have met the minimum TIG requirements for promotion from E-4 to E-5. In most cases you are receiving a career opportunity advancement selection, this will bypass the promotion list. You should be laterally promoted to CPL on the date you begin your AGR tour, and promoted to E-5 when you have met all the criteria, if you have already met the criteria your DOR for SGT should be the same as the date your AGR tour begins.