Isa's 2017/18

Basically, correct.

I assume there is no benefit opening the isa pre April this year with a 4k investment that could then transfer into a Lisa post April enabling that fund to then be 8k?

If you had a normal ISA from this year that you transferred into a LISA once the LISA product is launched, the value transferred in will count towards that year's £4k allowance of what you can get a bonus paid on.

But that transferred-in bit wouldn't count towards the £20k maximum of new money that you can put into ISAs in total during 2017/18, because it's not new money from outside an ISA altogether, it would just be seen as old 2015/16 ISA money which you're moving between ISA types and won't affect the £20k remaining annual allowance for 2017/8.

The exception is if you put money into a Help To Buy ISA this year (if you qualify for one), and transfer *that* into the LISA during 2017/8. The HTB is a special type of ISA which can pay a 25% bonus if you qualify for buying a property with it. The government has agreed that if you transfer 2015/16(and prior) HTB ISA money into a LISA in 2017/18 you can have the bonus on it *in addition* to the bonus on the £4k direct LISA contributions you make that year.

HTB ISAs have restrictions on how much you can put into them on opening and ongoing each month. The most you could get in during this tax year if you don't yet have one (but qualify to get one, having never owned any residential property) would be £1k at opening plus £200 for each of Feb, Mar, Apr, giving you £1600 at end of this tax year. You could then transfer it into the LISA from April in addition to the £4k of new money, giving you £5600 in the LISA to get a bonus on and £16000 spare allowance for 2017/18 non-LISA contributions that get moved over to the LISA for bonuses in subsequent tax years