Mark 4 is a collection of parables from domestic and farming situations. The parable of the sower is a healthy reminder that as disciples of Jesus all we can do is plant seeds -- to broadcast what we know and have experienced. Not all of those seeds will ever make it to the soil (some fall on the path) not all seeds will survive long (some fall on rocky soil and never put down roots) not all seeds will stay true to the faith (some fall among thorns and get choked off by the cares of this world) but some will fall on good soil and will reproduce. As Disciples of Jesus we have to remember to spread the word in every and all opportunities with the hope that some will take root -- remember others water, others fertilize but God gives the increase (1 Corinthians 3). We are not responsible for whether the seed grows or not. The law of averages would suggest that MORE planting in this kind of system will in fact produce more fruit in the long run.
Don't hide your light under a bushel -- if you are follower of Jesus be upfront about it. It is critical that the Christ followers are shining as bright as they can: through what they say (see above about seed planting) through what they do (you life may well be the only Bible some people ever read) and through who they are (integrity cannot be faked!).
Faith like a grain of mustard seed. The mustard seed was the smallest of the plant able seeds in the Middle East and, along the Galilee where it never frosts, mustard continues to grow from season to season. In their situation the mustard plant grows large enough for birds to build nests and make their homes. Point: it does not take a lot of faith to do great things for God it just takes faith.
It all ends with a wild story of Jesus and the 12 (and how many others?) on an overnight cruise across the Galilee only to get caught in a mighty storm. Jesus, as the Son of God, has no fear and is calmly sleeping. The Disciples are in full panic mode. They wake Jesus up and instead of being concerned for them he is angry that they had to wake him up. He calms the sea and stops the wind and there is a sudden calm. The disciples are (and this is typical of Mark) amazed and dumbfounded as to how Jesus could do this. The recurring theme here is that the Apostolic circle does not understand the TRUTH about Jesus until after the Resurrection.