Apple Watch Series 4 & Old Strapes

If you buy an Apple Watch by the Apple Store you can't buy only the watch, you buy the watch and a strap. Subsequently you can buy a strap that you want and if you find a strap of an older season you can use that. But remember that 38MM now is 40MM and 42MM now is 44MM.

It was clearly announced during the keynote that watch bands purchased for the older series will fit the new Series 4, so upgrading owners will be able to keep their accessory bands. The only issue as you mention is picking the appropriate width.