I just bought a used car with 4 winters still on it, the two in the front are not going to work for winter, but the two at back have 90% of the tread left. Can I take the good ones off and replace them with good used all seasons and leave the toast winters in the front, until winter?
If you think the winter tires won't last the following winter, then you should change them immediately. Winter tires will get decimated during summer conditions.
They are also much less effective in warmer condition compared to all seasons, so if you would not run on a tire for its intended purpose, you should not drive on it when it is in sub-optimal conditions. Braking distance is affected by the type of tires and braking is one of the best safety features on a car.
In this article from Bridgestone, they describe it in a little bit more detail. There's more information if you follow the link, but this is the important bit.
WHY YOU SHOULDN'T USE WINTER TIRES YEAR-ROUND
Winter tires are specially designed for cold temperatures and winter precipitation. Once it gets warm, you won't need deep tread depths to handle snow or biting edges for traction on ice. Here are some specific reasons why using winter tires year round is not recommended.
Faster wear on warm, dry pavement - the tread rubber of winter tires is considerably more flexible than that of all season and summer tires. That same pliable tread rubber that adds traction in winter will wear down quickly in warm temperatures. Summer and all-season tires are built to withstand warm temperatures, providing long wear life.
Decreased performance - In warm weather, winter tires won't provide the same handling capabilities as summer or all-season tires. Imagine if you needed to make a quick maneuver and your tires were soft and squishy. You won't get the crisp response from a winter tire in warm weather. Winter tires need that flexibilitity to handle ice and snow, but it's not as useful in warm weather.
When tires are replaced in pairs in situations like these, the new tires should always be installed on the rear axle and the partially worn tires moved to the front. New tires on the rear axle help the driver more easily maintain control on wet roads since deeper treaded tires are better at resisting hydroplaning.
Will it work? Yeah...
The downside is that you'll cook the winter tires in the summer, and destroy them. I'd recommend getting some 3-season tires (4 of them) for the Summer, and when winter comes buy two new winter tires. You'll then have two complete sets of tires which are ideal for the uses you'll put them to, and you'll get a much longer life out of them.