Katy - you have resurrected an old thread, but I will try to help. I am wondering if you took the first three tests all on the same day, and then the forth test the following day, or if you took the four tests on consequetive days?
Most importantly with the clearblue tests I am glad to read you read the test within the 10mins, if you did not then you need to be aware that these tests very frequently have lines appear after the time limit, they are called evaporation lines. The lines can look very real, but are only real if the lines are seen before the time limit.
Next, If you have taken the four tests over the four days, and the lines have gradually become lighter over four days, then I would suspect you had what is called a chemical pregnancy. Where the pregnancy fails to implant properly and progress, this is extremely common and it is being seem more often these days, not because it is happening more, but because women are testing before their period is due with the more sensitive tests available.
If you took three tests on one day and one the following day, then it is too soon to say whether it is a chemical pregnancy or not, the amount of dye in the test and the concentration of the urine, make a huge difference in the early days, it can go either way. Really the best thing to do is to wait until your period is due before testing again. Early testing is expensive, if you really can't wait then at least order cheap tests off ebay, to save yourself the expense.
I hope that it turns out the way you want it to and that you get a clear bfp soon.