Boil-in-bag will work, but for 4 shoulders -- I'd use the oven. You'd be boiling bags forever to reheat that much meat.
Your mileage may vary!
Southern Brethren BBQ Competition Team
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KingFisher Gator Rotisserie cooker, WSM, Stainless 5 burner with IR gas grill, Turkey Fryer, Weber JD Commemorative grill, slightly used Bandera chamber for future use as a cold smoker, Masterbuilt 40" insulated ELECTRIC smoker