Product review (5 stars): My revenge on the little winged rats....
You could pay more for an entire bird proof vent assembly and all the work involved in removing your old one and installing the new.  Maybe you can do it cheaper by fabricating something out of hardware cloth that you hope that will keep the birds and mice out of your vents.  For me, this bird guard was a no brainer.  Our home has three 4" vents installed just above a 2" x 12" exposed frame member.  The starlings just love to use this as a perch while they go in and out of the vents.  The bird dirt (aka poop) feathers, noise, and other damage had to stop.  The Bird Guards are easy to install, however if you're going up 2 stories you might want to have someone braver and more competent up there doing the installation work.  We did have to trim 1/2" off the length - for reference our vents are a Home Depot item; the guards might be a perfect fit for your 4" vent.  Careful removing the vent flaps, they can be brittle due to age and heat.  Our former bird tenants are now gone;  no more of their messes, noise or damage to our house.  They're off visiting other less well defended homeowners.