Can I use LT5400 as a single resistor? Can I connect the 4 resistor in series?
Yes

It is only four resistors in a package, so you can can do anything you'd normally do with four resistors, including connect them in series to get a higher resistance.

It's not often done, because it's rarely cost-effective.  An LT5400 is about £10, and a single precision resistor is nearer to £2.  The reason the LT5400 is more expensive is that the four resistors it contains are very carefully matched, both in resistance and in temperature coefficient. But if you just connect them in series, then that's a bit wasted.