In a market sense, you would deduct around 10K for the loss of a bedroom.
However, it is important that your home be as functional as possible while you are there (Especially if you are 5 years + away from needing to sell).
If you do a nice job with the conversion, I would almost expect a wash in terms of value. Also , when your home is being compared to other homes in the neighborhood, yours will stand out as being more functional and updated.
Typically it's a bad idea to convert a 4 bedroom house to a 3 bedroom house but there are instances where it might make sense to make the conversion. You might want to consider converting 2 bedrooms into one if the bedrooms are very small and you are creating a larger more functional bedroom, possibly with an en-suite bathroom. Often times buyers will see a 4th bedroom that is small and say "this would make a great office" showing you that your 4th bedroom does not actually make sense as a bedroom and should be converted. If your house is located in a neighborhood that is predominately 4 bedroom homes it may drastically hurt the marketability/value of your home if you remove one of the bedrooms so make sure to carefully consider your options.
I would be glad to discuss this matter further so feel free to contact me. It's a bit difficult to make a decision about your property without knowing the layout of the house, total room count, and what similar sized homes in your neighborhood have for a bedroom count.
Best regards, Dave
DRE Lic. # 01796995
Tahoe Riverfront Realty, Inc.
530-583-3483 ext 7