There are upgrade licenses and full licenses.
The upgrade licenses upgrade to a new release max. as many devices covered by the full AND by the upgrade license. I.e. having a 250 nodes full license and a 500 nodes upgrade license upgrades 250 nodes.
Since rel. 5.1 a 100 nodes license is upgraded to 128 nodes, a 250 nodes to 256 and a 500 nodes to 512.
With an upgrade license to rel. 5.1 together with the existing 250 nodes license of rel. 4.4 you get 256 nodes supported. Buying an additional full license of rel. 5.1 you get additional 256 nodes. Makes altogether 512 nodes.
Because a full license of rel. 5.1 will also run release 4.4, buying a 256 full license now together with the existing 250 nodes license of rel. 4.4 supports 250 + 256 = 506 nodes running rel. 4.4.
To upgrade from rel. 4.4 to 5.1 you need only the upgrade license of 5.1, i.e. you do not need first to upgrade to 5.0.