Im in my basement measuring my 5 gallon carboy and what looks to me like you are right at where the carboy begins to taper to the neck from the biggest part which tells me that you are approx. 1/2 to 5/8 of a gallon short of being correct. Right now since it is still fermenting there is no need to top off your carboy as gases are protecting your wine from oxidation. As to what to do when it is done then there are a few options.
1) you could sweeten back your wine by taking a fruit juice such as Plum smart which s a juice in your grocery store. Ive done this before to add more flavor. What I do is to buy 2 of these juices, put them in a pot on the stove and reduce them by simmering them both down to about 1/3 its original size at medium heat with NO LID so as that the water is evaporated out but the flavor and sweetness are concentrated. For your batch I wouldnt cook these juices since your abv will be so much and just add the juices straight into it so that it
WILL dilute the abv and bring it down and raise the level in your carboy. Remember that you have to add k-meta and sorbate to this batch once it has
STOPPED fermenting. You check this wine for stabilization by using your hydrometer a few days in a row to make sure that the SG is not changing. After it is stable and you have added the k-meat and sorbate you may add your juice and take a SG reading again. After a few days make sure it is not fermenting again by taking another SG reading. give it a few weeks and then you can egin to add fining agents or let time clear this wine. Please be careful when adding juice not to make it too sweet as this is very hard to undo!
2) The other option is to back sweeten after wine is stable by making a simple syrup. This method would be better if you are happy with the flavor profile and just want to make it a little sweeter which will bring out a little more flavor. we do this by taking 2 cups of sugar and putting this into 1 cup of boiling water to make sure it is dissolved and slowly add this to our stable wine which has been sorbated and k-mete'd to prevent re-fermentation. Again be careful not to over sweeten. This probably is not the best solution to your batch since you are needing quite a bit to top off.
3) This is the best option most of the time but again in your situation where your starting SG was so high it needs to be brought down. When making fruit wines you should have an assortment of different size carboys such as 3 gallon with multifit bung
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and 1 gallon jug with
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and wine bottles with
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to hold any left overs for topping up after you have racked.