My biological maternal grandmother died when my mother was 4. My grandfather remarried right away to the woman I considered my grandmother- in fact, for much of my life I thought she was my mother's real mother. When I found out otherwise it didn't affect how I felt about her but I do often wish I could have known my real maternal grandmother. I must say, though, that neither of them meant as much as my paternal grandmother or both of my grandfathers.
All of them are gone now, but I feel very fortunate to have had them all.