By the time Jesus describes the fourth soil He has already told you everything you need to know about why the other three fail.
The wayside person heard but could not grasp — the word arrived in a territory where understanding had not yet been opened, and before it could take root the enemy snatched it. The rocky ground person received with genuine joy but never allowed the word to complete its journey from the ear to the heart's deposit — emotion fired but meditation never crystallised it into conviction. The thorny ground person had real faith, real growth, but competing agreements were running alongside the word and making equal claim on the heart's allegiance — the word was present but it was not governing.
Three soils. Three different points of failure. Three different places where the four-stage process broke down before it could complete.
And then Jesus says this:
But these are the ones sown on good ground — those who hear the word, accept it, and bear fruit: some thirtyfold, some sixty, and some a hundred.