There are really three issues here - and I think you need to evaluate each separately.
The first is your inlaws capability to care for your daughter for a week. Assuming they're both young, healthy & properly cautious that's probably a yes. Assuming they will always hold her hand crossing the street, in a public place, etc. If they are less than capable (not healthy enough to keep up with an active 4 yr old, forgot everything they used to know about raising kids, etc) then this is a no.
The second is your child's ability to be away from you for a week. This is a tougher thing. Some kids, even if only cared for by their mom are ready, willing and able to go on an adventure with their grandparents. Others would be bereft without the usual person (mom) putting them to bed, cutting their hot dog just the right way, etc. Is there anough time for your daughter to spend a long weekend with your inlaws to kind of test it out? You may not be able to really figure this one out until that week comes. Most kids will get a little homesick (I did at age 10 when i went to summer camp for a week - but it passed quickly) - but grandmas and grandpa may be able to soothe that pretty quickly with a favroite book from home, a call to you (face time on your iphone, skype?). If they're doing something every day your daughter may be too tired to get homesick as she'll just fall asleep each night.
Finally, are you ready to spend a week without her? THAT's probably the sticking quesion and I undrestand your feelings about that completely! Until my daughter was 12 I never spent more than one night away from her (and those nights away were sleepovers at one of my sibling's homes as a cousin sleepover).
My inlaws were nevery healthy enough to have my kids on any kind of overnight outing, and even when they'd take them out to dinner at Friendlys or out to get a manicure or something like that I'd hear comments from my older child afterwards about the younger one dashing out in traffic (grandpa wasn't holding his hand). But that was a result of their health problems (grandpa had a lung disease that made him short of breath, etc.) But, my sisters who are both grandparents to preschoolers, are young (late 50s) and healthy and I could easily see them being able to provide excellent care for their grandkids.
Had my kids had young healthy grandparents I would have had to face the same thing you're facing - can I be away from my child for a week and feel confident that they will be OK without me? Assuming the answers to the first two questions are fine - then only you can answer this last question.
It could be a nice time for your and your husband too, to reconnect and remember why you love eachother
My final word, is this - if your inlaws will be able to properly handle the childcare needs of your child - I think it would be a great time for your child to really deepen her relationship with her grandparents. How wonderful for children to have this other circle of people who love them dearly and want to give them their unconditional adoration and blessings. Having this kind of connection to their extended family is so good for children - it gives them a security that goes beyond just mom and dad. It shows them that they're part of this continuum of the generations of their family. It give them a history and a future.
But YOU have to be comfortable with it. As the mom I think you get final say, and there should be no pressure from your husband or your inlaws.
PS - as for some posts here commenting on your husband have never bathed your child until last week - I can relate. My husband was the one to crawl around on the floor with them, rough house, take them go-carting, the playground, etc. He also took them to school, changed diapers, cleaned up after dinner, etc. BUT - he didn't feel comfortable with the bath thing - the wet wiggley child, drowning concerns? I'm not sure why. So don't get upset about that. I have all these photos of my husband and kids "doing life" together. Giving a bath is not the measurement of parenting skills.