The L3 is a single feed (not single ended) differential tube amp. The T4 is a parafeed differential hybrid amp. The T5 will be, if it every gets done, a more powerful version of the T4 that also does away with the parafeed cap.
The T4 was designed to be less expensive by reducing the power and by eliminating a bunch of assembly labor. But within its output limits I think it is 98% of the quality of the other amps. There are of course a few things you can do with a higher budget, but they are mostly gild on a lily.