Partner must have a void in either
spades or diamonds. If he has two trumps,
he can get his ruff later. Thus, you need
to assume that partner has exactly one trump.
If you decide to lead a small, lead your highest spade.
If you decide to lead a D, lead your lowest D.
The 4H opener places partner
with one of the following distributions:
4-8-0-1 (*)
3-9-0-1
0-8-4-1
0-9-3-1
(*) some players will not open 4H with
a four card spade suit.
10 card suits are too unlikely to consider.
The auction makes it unlikely that partner has
0-7-5-1. I'll ignore that as well.
As LHO doubled 4H, and RHO jumped to 6C
rather than investigate other strains,
clubs most likely are 6-3-3-1 across the table
- RHO should have at least six, and LHO might have
found some other bid with a C doubleton.
RHO can also be expected to have at most
four diamonds and at most four spades.
The next thing to consider is RHO's heart control.
This could in theory be any of
o the HA
o a H void
o a H singleton
o four or more hearts, and thus expectation
that his partner has at most a singleton.
A H void can be ruled out. That would place RHO with
3-0-4-6 or 4-0-3-6, and thus LHO with at most
a doubleton in either spades or diamonds.
RHO having a H singleton is possible only if partner has
3-9-0-1, or 0-9-1-3, or that ten card suit after all.
Even then, it would require unusually aggressive bidding by
RHO to jump into 6C with the CA missing, the HA missing, and
some holes to fill in D and S, too.
It thus is likely that RHO holds the HA, and at least
a doubleton heart.
However, none of this helps decide whether
partner's void is in S or in D.
Thomas