Assume that any number of companies may be merged at a time. We must decide how much to care about timing. For example, do $ \begin{aligned} &a,b,c,d\rightarrow\{a,b\},\{c,d\}\rightarrow\{a,b,c,d\}\\ &a,b,c,d\rightarrow\{a,b\},c,d\rightarrow\{a,b\},\{c,d\}\rightarrow\{a,b,c,d\}\\ &a,b,c,d\rightarrow a,b,\{c,d\}\rightarrow\{a,b\},\{c,d\}\rightarrow\{a,b,c,d\}\\ \end{aligned} $ count as one outcome, two outcomes, or three outcomes? If we care only about which groups get merged, but not about the timing, counting these as one outcome makes sense. But if timing is important, counting these as three outcomes makes sense. One might also disallow the first of the three, assuming that two mergers never take place exactly simultaneously, in which case only the last two would count.
If we assume that timing is unimportant, so that the scenarios above count as one outcome, then for four companies we have $ \begin{aligned} &a,b,c,d\rightarrow\{a,b,c,d\}\\ &a,b,c,d\rightarrow\{a,b,c\},d\rightarrow\{a,b,c,d\}\quad\text{(plus three others)}\\ &a,b,c,d\rightarrow\{a,b\},c,d\rightarrow\{a,b,c,d\}\quad\text{(plus five others)}\\ &a,b,c,d\rightarrow\{a,b\},c,d\rightarrow\{a,b,c\},d\rightarrow\{a,b,c,d\}\quad\text{(plus 11 others)}\\ &a,b,c,d\rightarrow\{a,b\},\{c,d\}\rightarrow\{a,b,c,d\}\quad\text{(plus two others),} \end{aligned} $ which is a total of $26$ outcomes. This is the result in the OEIS link relating to phylogenetic trees in Brian Scott's answer. If we consider timing to be important, then the three outcomes in the last line turn into nine outcomes, and the result is $32$ outcomes total. This is the correct answer for the partition model described, but not actually implemented, in Brian Scott's answer. (For a full discussion of the partition model, see Lengyel's constant; for the sequence, see A005121.) If we forbid simultaneous mergers, then we reduce the total by $3$, leaving $29$ total outcomes, as in Christian Blatter's answer.
Added: There was a question in the comments to Brian Scott's answer about the derivation of the recurrence $ \begin{aligned} M(n+1)=&(n+2)M(n)+2\sum_{k=2}^{n-1}\binom{n}{k}M(k)M(n-k+1)\\ =&nM(n)+2\sum_{k=2}^n\binom{n}{k}M(k)M(n-k+1) \end{aligned} $ for the number of outcomes in the phylogenetic tree model. The idea is to partition the set of merger histories according to the size, which we will denote by $k+1$, of the first conglomerate to contain firm $n+1$. Note that $k$ ranges from $1$ to $n$.
If $k\ge2$ then there are two scenarios, distinguished by the manner in which firm $n+1$ gets merged with the other $k$ firms. In Scenario I, the other $k$ firms first merge into a single conglomerate, and then firm $n+1$ merges with it. In Scenario II, the other $k$ firms first merge into two or more conglomerates, and then firm $n+1$ and these conglomerates all merge in a single step. Either scenario can take place in $M(k)$ ways, for a total of $2M(k)$ ways.
Once firm $n+1$ has joined a conglomerate, that conglomerate must then be merged, along with the $n-k$ remaining firms, into a single conglomerate. This can take place in $M(n-k+1)$ ways. As there are $\binom{n}{k}$ ways to choose the $k$ firms with which firm $n+1$ forms its first conglomerate, there are a total of $2\binom{n}{k}M(k)M(n-k+1)$ outcomes for a given $k\ge2$. The only difference when $k=1$ is that Scenario II cannot occur. Hence the coefficient $2$ becomes $1$.
The term $(n+2)M(n)$ is a combination of the $k=1$ and $k=n$ terms of the sum.
2nd addition: For completeness, I record the recurrence for the partition model here as well. Let $Z(n)$ be the number of ways in which $n$ firms may be merged. The initial set of $n$ firms may be partitioned into $k$ non-empty parts in $S(n,k)$ ways, where $S(n,k)$ is the Stirling number of the second kind. The set of $k$ parts represent the set of firms present after the first round of mergers. Since at least one merger must occur in each round, we have $1\le k\le n-1$. The new set of firms may be merged in $Z(k)$ ways. From this we get $ Z(n)=\sum_{k=1}^{n-1}S(n,k)Z(k). $