I start off with ~7gallons and wind up with ~5.5 gallons, however I do a 90 minute boil!
My kettle is about 17 inches in diameter. I calculate for a 5.5 gallon batch due to the loss of beer left in the fermenter when racking to the bottling bucket!
A lot of what effects the loss of liquid during the boil is size of pot, the more surface area boiling the more loss of liquid to evaporation.
1.75 gallons loss sounds about right(well technically .75 since you started out with 4 gallons), 1 downside to a smaller kettle is that you really cannot do a full boil. For you to get 5 gallons you have to boil at least 6.75 gallons.
Of course, topping off to 5 gallons is not hard to do either.:cross:
Good luck and keep on brewing:rockin: