Remember that the solution is actually HAuCl4. For the 1% solution that Salt Lake Metals sells, the pH should theoretically be 1.3.
Their solution is 1% gold by weight, which makes it .05 Molar.
Since there is a one to one relationship between gold and hydrogen in the HAuCl4 molecule, the H+ is also 0.05 Molar
pH is the negative logarithm of the hydrogen ion concentration, so log10 of 0.05 equal -1.3 making the ph 1.3
When diluted to make colloidal gold using 4 ml of the gold chloride solution per liter (40 ppm) the pH should then be 3.7
Each ml of the gold chloride contains 10 mg of gold, so 4 ml is 40 milligrams
40 milligrams of gold is .0002 moles of gold and .0002 moles of Hydrogen ions.
Then log10 of 0.0002 = -3.69 for a pH of 3.69.
Of course it wont be exact because the starting pH of the water will not be exactly 7, and there may be some residual hydrochloric acid in the gold chloride solution. But this should give you a really good guess.