Well, I would not rate them a 4 on the hardiness scale, maybe a 6. Bigleaf hydrangea can be grown in Zone 5 but it will usually suffer from die-back of stems unless you have a mild winter or unless you winter protect it. The plant roots will be fine so it will regenerate the stems. Then, if it blooms on old wood, it will not have blooms. If it blooms on old and new wood, the old wood flower buds will not survive but the new wood flower buds will produce blooms later in the growing season, like late June or July. Look for the Together & Ever Series, the Lat's Dance Series, etc. Some people have complained of the Original Endless Summer having blooming issues on the remontant buds so I would try the others first if you find them. Blushing Bride, part of the ES Series, seems to do fine though.
When adding winter protection, use dried out leaves or mulch if you do not have enough leaves. If the winter protection does not work or does not work well, tweak it on the second year. For example, if you use the chicken wire method, put more distance (and more mulch/leaves) between the end of the stems and the chicken wire. For stems that are growing upwards, add more distance between the end of the stems and the cardboard. Invisible nascent flower stems develop near the end of the stems. Usually the last one blooms if there is no winter injury. The others can be considered as 'back ups but if the stem dies, all of them dry out and die. For sure, the only way to obtain bloomage -dependably- in Zone 5 would be to use winter protection. And since late frosts could kill the flower buds when the plant awakens, consider a rebloomer mophead or lacecap.
One bad caveat in all this... some of the florist hydrangeas tend not to survive Zone 5 at all. They are not bred for intense winters. Like some of the Teller Hybrids. If you plant them, I wonder if it would be best to do that in the Spring 2016 so the roots have a chance to develop a good root system. You could keep it in a pot with good potting mix and plant them later in 2016. Store in a shed or garage and water it once a week or once every 2 weeks.
For dependable bloomage, I would look at the paniculatas or arborescens hydrangeas. Oakleafs are hardy to Zone 5 too. The first two bloom on new wood, which makes them slightly more "dependable" than the old wood oakleaf hydrangeas.
The last two years, mopheads and lacecaps have had a terrible time with winter. In some places, the temps went down lower than normal. In places like mine, the temperature swings re-awakened the shrubs just as a new cold snap hit so there was both die-back and flower bud issues. So, some oakleafs failed to bloom at all last year.