First, the context of the verse: 

This surah was revealed after a major battle (Uhud).  Many men were killed and many women widowed. 

Here are the verse related to polygamy and my interpretation:

[an-Nisa' 4:2] Give unto orphans their wealth. Exchange not the good for the bad (in your management thereof) nor absorb their wealth into your own wealth. Lo! that would be a great sin.

This verse is telling the men to give the orphans back their wealth...after they have kept it under their guardianship.  They are instructed not to be unfair ("exchange not the good for the bad") and to not absorb the wealth of the orphans into that of their own.  In other words, take care of them...and make sure you give them back exactly what you had been holding for them.

[an-Nisa' 4:3] And if ye fear that ye will not deal fairly by the orphans, marry of the women, who seem good to you, two or three or four; and if ye fear that ye cannot do justice (to so many) then one (only) or (the captives) that your right hands possess. Thus it is more likely that ye will not do injustice.

Now, the verse tells the men what they should do if they feel they might not be able to deal fairly with the orphans...as outlined in 4:2.  They can marry "of the women which seem good to you".  So, they are free to marry the women..if they feel they cannot be just to the orphans (All of this is happening because there were a lot of widows and orphans after the battle).  However, they are advised to be just and fair...one more time: If they cannot be just to more than one....marry only one.  These were all ways the men could help the situation.  It was a kind of 6th Arabian welfare system.

Note: One explanation of this passage mentioned that the Arabic "yatama", here translated as 'orphan'...refers only to children who have lost a father.  If the mother was still living, the child was still considered an orphan.  I don't know if this is true or not..but if it is...it puts a somewhat different spin on the passage.  It makes it sound as if the men were to marry the mothers of the orphans rather than just accept guardianship of the orphans...if they didn't feel they would be able to be just.

[an-Nisa' 4:4] And give unto the women (whom ye marry) free gift of their marriage portions; but if they of their own accord remit unto you a part thereof, then ye are welcome to absorb it (in your wealth).

This is telling them to give the women a dowry.  If the women return part of it...they can keep it.

[an-Nisa' 4:5] Give not unto the foolish (what is in) your (keeping of their) wealth, which Allah hath given you to maintain; but feed and clothe them from it, and speak kindly unto them.

The foolish here are the orphans spoken of in verse 2.  They are not to give them their money before they are responsible enough.  They are to use this money to feed and clothe them though.  Additionally, they are told to be kind to the orphans. 

[an-Nisa' 4:6] Prove orphans till they reach the marriageable age; then, if ye find them of sound judgment, deliver over unto them their fortune; and devour it not by squandering and in haste lest they should grow up Whoso (of the guardians) is rich, let him abstain generously (from taking of the property of orphans); and whoso is poor let him take thereof in reason (for his guardianship). And when ye deliver up their fortune unto orphans, have (the transaction) witnessed in their presence. Allah sufficeth as a Reckoner.

When the orphans are old enough...and the guardians find them "of sound judgment"..they are to "deliver over unto them their fortune".  In other words, they are to now give them their money back.  If the guardian was well off, he is asked to be more generous.  If someone is poor, they are allowed to take some of the money...within reason...for the guardianship.  Also, the money should be given back in the presense of witnesses.