Hi, I need some expert advice on IRA / 401K plan.

Background
I am an alien worker currently working on H1B. I recently joined a
company which offers 401K (after 6 months) and also I am in a
initiation process to file my Green Card. However I am planning to
return to my home country after 5-6 Yrs. I need to take a decision
before April 15 whether I should put some money in IRA or not.

My understanding
My understanding is both IRA and 401K are tax differed and hence I
will have to pay taxes when I withdraw it more over I will even have
to pay a 10% fine if I decide to withdraw it before maturity. Also
there is a 5 Yrs vesting period to withdraw 401K money. My initial
plan was since 401K is free money why to leave it? Accordingly I had a
thought that I will have a Traditional IRA a/c for current year (Since
not eligible for 401K this year) and will put 4000$ in it. Next year
onwards I will put money in 401K and company will match certain % to
it. After initial 5-6 Yrs when I plan to go back I will rollover 401K
money into IRA and will leave the country. The following year (year
after leaving US) since I won't have any US income I can withdraw it
in phases to avoid tax?

Problem
However this leads to numbers of questions -
1. Whether my understanding is correct. Can I phase out the
withdrawals to avoid tax and/or penalty?
2. Will it make any difference if by that time (though looking at the
pace looks very hard) if I get my Green Card and then leave the
country ( I know I have to maintain the status but still will it make
any difference)
3. Do we pay taxes as per tax slab or it is a flat 30% tax for alien
workers?
4. Are there any issues while rollover?

In a totality does it make sense to have IRA/401K account? Or it will
be a loss business ?

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