Yeah it would be almost impossible to tell in some cases without an ultra sound. Some women, in very rare cases, can have a period even though they are pregnant, and some can have no signs of pregnancy at all. Sometimes even a test is inconclusive either because some freak of nature doesn't allow it to show up for some women in rare cases or because it was a bad test.
The signs of pregnancy can be signs of other things, so even if she did have symptoms of pregnancy, they could be for other reasons. The opposite is the same. She may have none at all too. Pregnancy can be a crazy thing.
As far as the stomach goes, she could have tightened her muscles just as you were feeling her stomach. At 4 months I doubt that you would be able to feel a hardness below her belly button without seeing that she has somewhat of a belly bump. I think it would be a little more obvious.
Of course the smart thing to do is for her to take a test, and you can offer to buy one for her and it doesn't hurt to try it. She could be lying to you or maybe she's living in denial. Or there could be nothing at all wrong.
If you can't do anything at all, pregnancy is a hard thing to hide as time goes on and you will find out eventually. The only thing bad about that is that if she is pregnant and doesn't know it, she's not getting the proper pre-natal care that she should.
