consistently there is a 4 degree difference between the master bedroom and the rest of the down stairs.
I don't see a way to add another regsiter, and the wife mentioned she saw a comercial on that ductless a/c stuff.
is this a good option?

how do you size it if you still have the regular a/c system in the house? or do you just size it for the room anyway?

do you mean just a hole between the bedroom and the rest of the house? or something else?
window shaker? just a window unit? Wanted something a bit cleaner than that, but will consider it.

Does the room have a return air duct, or is it done with just the crack under the door? Sometimes carpet is added which eliminates the gap, and when air can't vacate it pressurizes the room, which then minimizes the air flow, as forced air of course takes the path of least resistance––which means other rooms in this case.

Also, if the duct is on the floor it could be the cold air is hugging the floor. A ceiling fan would remedy that.

we have one of the ductless systems at the fire house and its ok. its no louder than a fan but just on the price point alone i wouldnt buy one just for one room. is your house a single story or 2, if 2 is your bedroom on the ground floor or second? get a HVAC guy to come out and check your system and see if it is large enough that you can just add a leg off the main duct and add a register in the bedroom. that would probably be the easiest/cheapest cause if he says yes the system is GTG then you can add the register and run the duct work yourself. its an hour job at most and shouldnt cost more than $50-75 in parts.

Go see if there is a damper in the supply duct to the upstairs. If the system is downstairs there is probably a duct that runs to the upstairs that is next to the furnace. Make sure it is open all the way. Also make sure your air filter is clean and the outdoor unit is not plugged up with lint or other debris.

your registers arent in the floor on the first floor? the outside unit for the ductless systems arent that bad. the one we have is about the size of a large luggage bag and runs 2 of the large wall mount heads.

Yup, this is a common issue. If you have positive air pressure in the room because air isn't escaping, it will find an easier way out, like another vent. But, it sounds like this isn't your issue.

Another option would be to put a powered vent in a wall/ceiling to vent into another area of the house, thus pulling conditioned air into the room. Don't push the air into the master, exhaust it. You don't want to work against the existing duct work when the AC is running. I don't know where your stack is (interior or perimeter wall), or if your air handler or duct work is in the attic, so I couldn't recommend a powered vent attached to an existing duct that could be heating up between cycles.

It is a system in the attic.
I will check again, but don't think there are dampers on this one. or didn't find any when we first moved in.

the vents are all celing mounted here. slab not pier and beam.

like the last post says.......first check the feed to your vent.........see how many vents are pulling off that trunk line

if there are other vents off that line, close the damper on some of them and it will push more to your room

also, if you have never been in the attic to see the trunk line and vent lines........you need to......look for damage caused by anything from storage boxes crushing a line to squirrel or wasp damage

also, its not that uncommon for a few degree differnce if that room is facing west - thereby being in the sun for most of the day........if thats the case, consider your blinds, curtains and such, even tinting the windows.

a modern ceiling fan running on medium speed (get reversible so you change direction in summer and winter) will do an amazing job of moving cold air around and hot air down

an odd thing but something you should consider, get a humidity measure.....radio shack or walmart has small ones.........check humidity in that room, attic and other rooms.......it can indicate too much moisture (which makes heat feel worse).....if so, your ducts may not be insulated well, or a host of other issues.