"My husband is worried that the permit will not pass for our renovation because even though our 4th room isn't a bedroom, the state/city will count it. "

More than likely they absolutely will count it as a bedroom since it is one now.

That would probably pass here, all you can do is submit the paperwork. It will depend on your local regulations. Visit your building/planning/health dept. and talk to a secretary, they'll know better than anyone on the internet.

I would think that the contractor or when inspector comes, the tank‘s size will probably be questioned. Therefore you may want to find out if it’s considered a bedroom with only a window. As well, down the road will you be selling as a 3 or 4 bedroom, so there could be an issue then.

^^^ Right, that's exactly the concern. Septic tank sizes are based upon the number of bedrooms. A closet or whatever won't even be the issue, it's the fact that it is registered with the County as a 3 bedroom. It is going to depend on how much of a "hardass" the inspector is.

So silly if you think about it a 4 bedroom house where only adults live and use 2 bedrooms will still need a bigger septic IMO how many people not how many bedrooms would make more sense .

^^Since houses change owners several times over the lifespan of a septic tank, they can't go by number of people. Say the OP sells it to a family of 6, which a 4 bedroom house would work fine for. If the tank was only rated for 2 people, then it would fail very quickly. Or wouldn't pass an inspection for a mortgage.

Ack I know! This is such a delimma. To take away a window in a room to make it constitute an office and not a bedroom.

The house has a 4 bedroom septic tank but it’s recorded as a 3 bedroom septic system.

I would not remove the window. I would call the auditor, or whoever is in charge. If you do in fact have a septic tank that is “up to code” for a 4 bedroom house, then I would request that it be reflected as such. I don’t know what the exact technical term is, but you can challenge what they have it recorded as.

(my house was built in 1968. 1968 codes required only a 500 gallon septic for 2 bedroom homes. Current code states that 750 is the min. If your tank was sized for the house when it was built, it may not meet current 4 bedroom mins. Or it only meets the requirements of a 3, which is why it was recorded as such)

If the office could be opened up more to the adjacent room ....no door just a wider opening... perhaps that would take it out of the bedroom category?

This article explains how you could work around this situation. You could have a deed restriction placed where your house can only be listed for sale as a 3 bedroom, even though you technically will have 4.

https://www.maxrealestateexposure.com/bedroom-misrepresentation-septic-systems/

Where I live, no closet, no bedroom.

Yes, septic systems are rated by the number of bedrooms, not number of bathrooms or number of people. That's how it is, you can't change that. The size of the septic tank is only one factor, the size of your leach field is the other factor and depending on your site and soil, it is the more expensive factor to upgrade, if they will allow you to do that. Your building site and size may play a role here. I can't see how you can get a permit for your new bedroom unless you make enough changes to one of your old bedrooms so it no longer meets your local definition of a bedroom, and I don't mean just moving in office furniture. I don't know where these people live who reply that it's not a problem, it would be a very big problem here on Cape Cod if you tried that. Talk to your building inspector for the most accurate information.