Not comparable

Because there is extra information (the presence of a biddable heart suit) which is not shown in the 1NT bid
North will have to pass throughout

If you check Law 23A for the definition of the comparable call:

A. Definition

A call that replaces a withdrawn call is a comparable call, if it:

1. has the same or similar meaning as that attributable to the withdrawn call, or

2. defines a subset of the possible meanings attributable to the withdrawn call, or

3. has the same purpose (e.g. an asking bid or a relay) as that attributable to the withdrawn call. 

As Nick commented:

"hands that bid 1H" are 4+ hearts and 6+ points (broadly speaking).

"hands that bid 1N' are ~6-9 points and a spade stopper (broadly speaking).

Even though 6-9 is a subset of 6+, the difference in suit definition (spade stopper vs natural hearts) means that these are not analogous bids.

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A 2H bid (whether it's a negative free-bid, or a positive free-bid) would define a subset of possible meanings, and thus be fine under 23 A 2. 
Showing 5+H is more specific than showing 4+H, and either having a range of ~5-10, or a range of ~10+ pts is a tighter definition than just 6+.

A negative double of 1S typically shows values to respond (~6+ pts), and tends to strong imply at least 4Hs. So this would also be a comparable call, despite not really defining a subset of the possible hands.

HI, I am training some directors at our club and one of them asked me this question :  

On a bidding, where a pair has bid to 4H and partner asks "4C" ( meaning Ace ask) not accepted by opps. Would 4NT Blackwood  be acceptable as a comparable call ?

Thank you for your opinion.

Happy New Year

Helene Labreche

YES it is comparable

That is an example of a replacement call with  "the same purpose"

Unless (highly unlikely) there is doubt as to whether 4C was intended as ace asking