Making this play against a dealer's four leaves a window of opportunity for the 7-10 effect (the ten's likely but the seven's a don't know for HiLo). If you believe that if it can happen it will happen then this is a high risk play, bearing in mind you'll have multiple max bets out. I would think the TC would have to be around +8 or higher for this to be advantageous? Again a matter of risk and return. I think there is a big shift in risk going from playing against a five to a four, three or two for this reason.
Although I have splitting tens and doubling A9 against a five in my armoury, I've never yet had the opportunity to play them.
Congratulations by the way. How much did you take?