Assualt in the 4th Degree is gross misdemeanors and is punishable by up to a year in jail and/or up to a $5000 fine. As to Maliscious Mischeif, if the property in question revieved over fifty dollars in damage, then the above punishments would also apply. If the damage was in an amount equal to or less than fifty dollars, than it would be a simple misdemeanor and therefore punishable by up to 90 days in jail and/or a $1000 fine. As to your immigration issues, if you were convicted of either of these charges, you may be subject to deportation if the crimes were determined to be "crimes of moral terpitude." You should consult with an immigration attorney on that issue to determine what would be the best course of action in your case.
Mr. Kiviat mentioned the maximum possible sentences on these charges. On first offenses, however, the court does not usually impose a jail sentence anywhere near the maximum - depending on the circumstances, of course. Whether you can get a plea bargain from the prosecutor that will reduce the severity of the charges and allow you to avoid probable immigration consequences depends on the individual circumstances of your case.
If these charges are classified as domestic violence, that would be an additional basis for immigration consequences. A conviction for an "aggravated felony" under immigration law would also likely be problematic. Assault IV can be considered an aggravated felony, even though it's a gross misdemeanor under Washington law. Hiring a skilled defense attorney to do a careful analysis of your case and put up a good fight may help you to avoid automatic immigration consequences. Staying out of jail can also be important to avoid having an immigration hold placed on you.
It would be a good idea to consult with an attorney who has experience working with immigration issues and who will obtain the advice of an immigration expert before recommending a resolution of your case. If you're thinking about hiring an attorney, be sure to ask about whether the attorney has experience handling cases with similar immigration issues. Don't be afraid to ask questions, interview the attorney and hire someone you feel comfortable with.
In addition to the possible jail, fine and immigration consequences the other two responses spoke of, if you are convicted of Assault in the Fourth Degree Domestic Violence you will lose your right to possess a firearm, possibly have a no contact order imposed and have to do Domestic Violence Batterer's Treatment.
Hire a good lawyer.