Chapter 4:30-34 Parable of the Mustard Seed

4:30 And He said, Whereunto shall we liken the kingdom of God? or with what comparison shall we compare it?
Whereunto shall we liken the kingdom of God? - How amiable is this carefulness of Jesus! How instructive to the preachers of His word! He is not solicitous to seek fine turns of eloquence to charm the minds of his auditors, nor to draw such descriptions and comparisons as may surprise them: but studies only to make Himself understood; to instruct to advantage; to give true ideas of faith and holiness; and to find out such expressions as may render necessary truths easy and intelligible to the meanest capacities. The very wisdom of God seems to be at a loss to find out expressions low enough for the slow apprehensions of men.

4:31 It is like a grain of mustard seed, which, when it is sown in the earth, is less than all the seeds that be in the earth:

It is like a grain of mustard seed - That is, the kingdom of God spoken of in the preceding verse, is like unto a grain of mustard seed; by which is meant it brings life or points to the kingdom of heaven, directs the way unto it, and shows what qualifies persons for it, and gives them a claim unto it.

4:32 But when it is sown, it groweth up, and becometh greater than all herbs, and shooteth out great branches; so that the fowls of the air may lodge under the shadow of it.

But when it is sown, it groweth up -So the Gospel, when it was preached, it spread notwithstanding all the opposition made against it by, the Jews and Gentiles: there was no stopping it; and shooteth out great branches, so that the fowls of the air may lodge under the shadow of it: by whom are meant, saints; such to whom the Gospel is come in power, and who have the grace of God wrought in their hearts, who are partakers of the heavenly calling: these come where the Gospel is preached, but where they lodge, abide, and continue, under the shadow of the Gospel.

4:33 And with many such parables spake He the word unto them, as they were able to hear it.

With many such parables - As they were able to hear - or to understand always suiting his teaching to the capacities of His hearers.

4:34 But without a parable spake He not unto them: and when they were alone, He expounded all things to His disciples. 

He expounded all things to His disciples - That they might be capable of instructing others. Outside hearers, those who do not come into close fellowship with the true disciples of Christ, have seldom more than a superficial knowledge of Divine things.

Chapter 4:26-29 Parable of the Seed

4:26 And He said, So is the kingdom of God, as if a man should cast seed into the ground;

So is the kingdom of God - This parable is mentioned only by Mark, a proof that Mark did not abridge Matthew.
The doctrine of the kingdom, received in a good and honest heart, is like seed sown by a man in his ground, properly prepared to receive it; for when he hath sown it, he sleeps and wakes day after day, and, looking on it, he sees it spring and grow up through the virtue of the earth in which it is sown, though he knows not how it doth so; and when he finds it ripe, he reaps it, and so receives the benefit of the sown seed. 

So is it here: the seed sown in the good and honest heart brings forth fruit with patience; and this fruit daily increaseth, though we know not how the Word and Spirit work that increase; and then Christ the husbandman, at the time of the harvest, gathers in this good seed into the kingdom of heaven.

4:27 And should sleep, and rise night and day, and the seed should spring and grow up, he knoweth not how.

And should sleep and rise night and day - That is, he should sleep by night, and rise by day; for so the words are obviously to be understood.
He knoweth not how - How a plant grows is a mystery in nature, which the wisest philosopher in the universe cannot fully explain.

4:28 For the earth bringeth forth fruit of herself; first the blade, then the ear, after that the full corn in the ear.

Bringeth forth - of herself - Αυτοματη. By its own energy, without either the influence or industry of man.
first the blade, then the ear, after that the full corn in the ear; which, as it very aptly describes the progress of the seed from first to last; so it very beautifully represents the gradual increase

--of the work of grace, 

--under the instrumentality of the word, 

--accompanied with the Spirit and power of God.

4:29 But when the fruit is brought forth, immediately he putteth in the sickle, because the harvest is come.

But when the fruit is brought forth - Unto perfection, and is fully ripe; signifying that when grace is brought to maturity, and faith is performed with power, and the good work begun is perfected; then, as the husbandman, immediately he putteth the sickle; and cuts it down, and gathers it in;
because the harvest is come; at death or at the end of the world, which the harvest represents: when all the elect of God are called by grace, and grace in them is brought to its perfection, and they have brought forth all the fruit they were ordained to bear, they will then be all gathered in; 

--either by Christ Himself who comes into His garden, 

and gathers His lilies by death; 

--or by the angels, the reapers, at the close of time, who will gather the elect from the four winds;