I don't know much about the condition of your various carb options, but I would not go to TR6 strombergs on a TR4, in fact I have been looking for TR4 4A strombergs off and on for my TR250 to replace the TR6 carbs now on the car.
The only problem I see with early strombergs is some parts, floats, etc, can be hard to find, but they were great on my 4a, really no different from SUs except the rubber diaphragm.
The later Strombergs are, in my opinion, the ones that gave Strombergs a bad name they are much more comlicated with more to get out of adjustment or go wrong, plus the mixture adjustment is harder and more limited.
What is the problem with your current strombergs?, sorry if it was already chronicled here and I missed it.