First off, it is impossible to spoil a four-month-old baby.
Second, every baby, like every child, is different. Some babies behave just like they came out of a textbook, other babies are harder to soothe and to settle. It isn't anything you're doing or not doing.
Third, there may be reasons your baby needs to be held like that. Both of mine did, and they turned into extremely active toddlers and very bright children. Both of them are Gifted. They're alert, adventurous, high-energy, sensitive children. As exhausting as it is, take it as a positive sign that your child cries for you and is connected to you like that. Some of these children are very reactive. It also could be reflux. When you hold a baby, you hold the baby's head higher than his stomach -- a naturally more soothing position for a reflux sufferer. Also, is your home on the chilly side? Could the baby want to be held because your home is chilly and you are a nice warm mommy? If it's reflux, he'll respond well if you talk to the pediatrician about what you do about that. Also, is this a new thing -- needing to be held all the time -- or is it just since you returned to work? Is it only you who will do, or will Daddy be able to soothe baby as well?
While you sort this all out, and in the event you are just blessed with a spirited child, I highly recommend the Baby Bjorn. It's kind of like a kangaroo pouch. I used mine until my youngest was a toddler! You have your hands free and can basically do everything but bend over (bending at the waist isn't good for your back anyway, though! LOL.) while holding baby.
Good luck. they're only babies once!!
Mommy to a 5-year-old and an 8-year-old.