The ESU v5.0 L has improved sound with up to 10 simultaneous 16-bit sound tracks vs v4.0 that supports 8 simultaneous 16-bit sound channels. The v5.0 also has improved motor speed control (apparently eliminating a faint hum at low speed when sound is off), and the O scale v5.0 L now has onboard keep alive capacitors where the v4.0 L has none. You had to purchase a separate large size keep alive module for around ~$50 to get the keep alive feature on a v4.0 L decoder.
Also, given that you said that you are looking at a v4.0 Select L, which was targeted at the OEM aftermarket plug-n-play installs, this Select did not contain all the features that the straight v4.0 L version had and was always priced slightly less than the straight v4.0 version to begin with. BTW, there is no longer a v5.0 Select, they dropped the select from the v5.0 product line, at least for now.
Therefore, for a mere $20 more, you are actually getting more for your money with the up-to-date v5.0 L version since it is still competitively priced with an apples-to-apples straight v4.0 L version, which was priced at the same price as the newer v5.0 version.
The only caveat I can see (as mentioned above) is if the v4.0 interface board already installed in the loco does not mate up with the new v5.0 L decoder (maybe someone else on here knows if the new v5.0 will mate up with the older v4.0 interface board). However, you will still get a new interface board with a new v5.0 L and if the v5.0 L does not work with the v4.0 interface board currently installed, then you will have to do some soldering and, I have to say, those pads are a bit small on the decoder interface board, even for an O scale sized decoder ,(I have done some v4.0 installs using the ESU L interface board) , so you do need a bit of a steady hand and some reasonably good soldering skills to not get a solder link across neighboring pads. Make sure to use a decent pencil tip iron or, even better, an electronics bench soldering station.
In the end, you really don't save any money going with the v4.0 vs. the v5,.0 when comparing features; however, you "may" save convenience of getting direct plug-n-play capability over the v5.0 if the older v4.0 interface board currently in the loco ends up not being compatible with the v5.0 L decoder header pin out. They look to be almost identical but not 100% sure if it will actually work. You could be an O scale trailblazer and let us know if you go and get a new v5.0 L 
.
Scott
