You might try a 10 hour lights on but work the timers so two come on for 8 hours then go off & the other two on for 8 hours but with a 2 hour period that all four are on in the middle.
IE 2 x on 12pm off 8pm ... 2 x on 6pm off 10pm.
That will give two hours of very intense light.
Although 2 x 54 tubes might cover the full lenth of your tank i dont know if it is really an upgrade - 4 x 39 = 156 - 2 x 54 = 108?
You can try that format to start with & adjust as necessary.
You dont really need 4 T5s as two will be adequate but if you can avoid the algae issues that might arise, then great!
I would sort out your fertilizer routine, make sure you carry out lots of water changes & plant plenty of stem plants to start with.
You'll find that the needs of your tank will change over time. In the beginning, 4x54 will grow more algae than plant. Over time though as you're plants grow in, you replace easy to grow "tank-breaker-inner" plants for demanding plants your lighting needs may change.
See my journal for the results 2x54 can bring in a 48" tank with fertz and CO2 as a ckpoint. Now, with a fairly mature tank and some plants that would benefit from a little more light, having a third or fourth bulb to turn on for a little while during the day might be good to change there appearance - redder, thicker...
But I don't need more growth. Eventually growth just means maintenance unless you are constantly changing things around. 4x54 can be good to grow thick carpets if you are going that route.
In my limited experience, 2x54 is plenty to grow almost anything. More would just be excess light which would create more fert and co2 demands resulting in more growth and therefore more maintenance. Without a lot of plants, fertz and CO2 that much light will just produce a LOT of algae.
Having said that though, it would be nice to have the flexibility to turn on some extra light for the more demadning plants for a short period each day, but it isn't necessary for healthy plants.