4 yr old only hyper when dad gets home

My 4 yr old son is hyperactive only when my husband gets home. He starts jumping around (sometimes dangerous), is very excited, cannot stop moving around,  will not listen to my husband, usually has to be told something many times or will get sent to time out chair, he becomes whinny and very emotional.He is great with me throughout the day (he does go to school from 9am-12;30pm) and he typically listens.  He sometimes misbehaves but is easy to control and gets back on track.  Around me he acts like what I think a 4 year should act like.  But when my husband gets home he changes so much that I can't stand to be home with them. We also have a 1yr old daughter.  My husband works 4 days a wk, 10 hrs a day.  He usually gets home by 4pm then I go to work until 9pm. My husband also just finished his Masters program and for the last year has put in alot of homework time in on the weekends.  November was his last class and now is with us on weekends. It has been this way since my son was born.  

My son is extremely close to my husband.  My husband is a great father and has always given our son quality time.  Plays with him indoors and out (hrs not minutes), reads to him, talks with him, teaches him. Even though my husbands time has been recently limited with our son the time spent is always quality.  Whatever time my husband has is given to our son.  Obviously his time with his parents is now shared with his sister.  My son constantly asks for his dad all day and can hardly wait for his dad to get home. That is why I can't understand the defiance or the mood change.  My husband does take our son to school and pick him up on Mondays(his day off).  My son knows this and will constantly make sure that my husband is picking him up and if for some reason I pick him up on Mondays he will get upset because he wants his dad.  Also, when he wakes up in the morning he always goes to see if my husband is in bed and then start to whine because he doesnt want his dad to be at work.

I believe some of the behavior issues stem from my husbands leniency in the first few years of our sons life.
I belive that I have required a certain behavior from my son since about the age of 2 where my husband didn't.  He figured that "things" were not a big deal because our son was so young. I believe he tended to spoil him a little too much and would allow him to be whinny to get what he wanted.  Now that he is 4 things are obviously not ok. I have been told by several people that  that my son behaves differently  with me than with my husband.  I am told he is a little more rambunctious and tends act spoiled or not listen when he is with my husband.  I also think that my son might be experiencing a little separation anxiety from my husband that cause my son to lash out at him.  We have asked his teachers and they say he is great at school.

I don't know what to do.  My husband is getting frustrated and short tempered. my son is upset offten with his dad.  My husband keeps bringing up that he believes he might be "hyperactive".  I don't belive this to be the case because like I said he only behaves like this when my husband gets home during the week.  Weekends are a little bit better.  Help!