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Is it okay to perform a Kruskal-Wallis on four unequal samples? Further to this, is there a subsequent pair-wise post-hoc suitable for two unequal sample sizes?

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Yes and Yes. Kruskal-Wallis analysis does not require equal sample size. In latest SPSS versions (from 18, if I remember correctly) there is a new nonparametrics procedure that performs pairwise comparisons with sig. adjustment, as well as step-down post-hoc method. Alternative would be to use very nice macro by Marta Garcia-Granero http://gjyp.nl/marta/

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@ttnphs Thanks so much! What is the new procedure in SPSS then? – Platypezid Jan 30 '12 at 19:12
Starting from version 18 you may notice new nonparametric menus (= NPTESTS command) and legacy nonparametric menus (= NONPAR TESTS command) under Nonparametric Tests. The new menus are ugly but there is an option for post-hoc which was absent in NONPAR TESTS. – ttnphns Jan 30 '12 at 19:25
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According to the formula for the Kruskal-Wallis test statistic, each group can have a different number of observations, so "yes".

Whether this is the best test or not I'm not sure - if you're still in doubt you'd need to post more details. But perhaps this is all you needed to know. Good luck!

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Post-hoc analyses after this rely on "pair-wise" comparison, so are Mann-Whitney tests okay to look at where the significances are? – Platypezid Jan 30 '12 at 18:53
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Yes, Mann-Whitney tests are the normal post-hoc test to use for a Kruskal-Wallis test.

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