When it was (potentially) too late I've noticed that I've used the 4L of water for mashing that I should've used for sparging - which was instead 3L. Could this be a problem? The end amount is the same. I've also noticed that the mashing temperature was very stable (probably due to higher water volume).
1.5 gallon batch?
You should fine. The higher grain to water ratio may result in a slightly high pH but you should still be well in the range for enzymes to do their work.