Yeah, test trips aren't very precise. Will trust that it is actually 4.0 and you have some color guide for measuring down to 3.4. You will want to add some form of acid which is a simple task.
The easiest thing to add is Acid Blend which is a mix of citric, malic, and tartaric acids typically. This is in powder form. You can add individual types of acids if you are looking for something specific but the blend is an easy way to get a good balance. Particularly as a newbie, you should start here.
Citric acid is what you might expect. It has a citrusy flavor - to me it is closer to lime than, say, blood orange or loquat.
Malic acid is what you'd taste in a green apple. Tart and crisp.
Tartaric acid is heavy in grapes - found in very few fruits so it's a distinctly wine-y acid. Grapes are primarily made up of malic and tartaric acids though the malic decreases as the grapes ripen leaving a more heavy concentration of tartaric.
With time, you can develop a deep appreciation for the various acids (there are more!) and choose/weigh according to the style you are trying to achieve.
For now, all you need to do is stir in small amounts and re-test. But be conservative because you cannot (easily) fix too much acid.
For a gallon batch, a 1/4 tsp at a time could nudge the needle a couple decimals. For a 5 or 6 gallon batch, you can probably start with a half or full tsp and test depending on how far you want to go. You will see how much the pH was impacted and expect to get the same movement with equal amounts more. So it becomes easy to baby-step down to 3.4.
Pro tip: make sure you mix super well. Very frustrating to see it go from 4.0 > 3.8 after a half tsp then from 3.8 > 3.1 with the next half tsp only because you did not mix well enough after the first dose and got a bad sample.
