Gauge and yarn substitution

I am subbing one strand 5 bulky for 2 strands worsted? And my gauge is 4 sts=one inch? Iam sooo confused…I am a semi- newbie lol

Also, I am making slippers…

Welcome!
Two strands of of worsted will give you a bulky to super bulky yarn. Best thing is to try a gauge swatch to see how many sts per inch you get with the bulky, whether you like the knit fabric and how close it is to 4sts/inch.